So how long has the helium 3 mining been going on on Luna and just how much longer can it likely continue? Does anyone have some estimates or numbers for such?

Well, the book leaves the dates somewhat vague. The general time table we use in our game is based on this; fusion technology hit its stride about halfway into construction of the beanstalk. The beanstalk was completed approximately 40 years ago. So it's been happening for at least 30-35 years.

As far as how much is left on the moon? The books don't give any clear idea.

Let's do some really vague, approximate math here, which is probably nowhere near close.

1. The concentration of Helium-3 in lunar regolith varies between 3 and 50 parts per billion ( https://en.wikipedia.org/wiki/Lunar_resources#Helium-3)
2. Let's assume the rough estimate of 150 tons of regolith for 1 gram of Helium-3
3. 3 grams of Helium produce about 493 MWh of energy ( https://en.wikipedia.org/wiki/Helium-3#Nuclear_fuel ), so 1 gram produces just over 164 MWh
4. Global energy consumption in 2018 was about 160,000 TWh ( https://ourworldindata.org/energy ).
5. Assuming that the future of Android has greatly upped our power consumption, let's take it to a hillarious estimate of 328,000 TWh (about double)
6. So, 328,000 TWh of power consumption, at 164 MWh per gram, that's 2 billion grams of Helium-3, or 2,000 tons of Helium-3 for a year worth of power
7. Following the quota established above, that would require 300 trillion tons of Regolith
8. At an average density of 1.5 g/cm³ ( https://curator.jsc.nasa.gov/lunar/letss/regolith.pdf ), that's 200,000 km³ of Regolith
9. The same source also lists it as a layer about 5 m thick, so we're look at 40,000,000 km² of lunar surface to excavate per year
10. Funnily enough, the moons total surface area is 38,000,000 km², so after 1 year of burning its Helium-3, we're out.

Now, this is obviously making some poor assumption on many accounts:
1) We have no idea how energy demand will look in the future of Android
2) We wouldn't power humanity on nuclear fusion alone; solar, wind and waterpower are still a thing
3) This is calculating with rough estimates, averages and data that NASA collected in the 60s

For anyone curious on the full calculation that went on:

The Worlds book actually estimates 110 tons of regolith per gram of He-3. They cite 0.01 parts per million of He-3 in the upper layer of lunar soil, which is 10 ppb to fit back into your calculations. So you are close to their expectation. Also it states that Weyland was banking on He-3 being the go to fuel of the world to get a return on their investment. Since they are now one of the (if not the) richest corporations on the planet and the Beanstalks primary import is He-3, I think we can assume it accounts for the majority of the power generation around the world. That said the world book goes into just how much solar power generation is used in space, on the moon and in the beanstalk operation. They also mention how the beanpods' downward travel also offsets some of the Beanstalks power requirements. So they are clearly generating power other ways to off load the fusion power requirement.

Worlds of Android concretely says thousands of years worth of energy can be taken from the Moon before the reserves are tapped out. They expect they'll be mining He-3 out of gas giants in the system long before then, and people are working on that process now.

So Worlds Of Android had the answer, but can we make sense of it? This was all taken from page 107 and 108 for anyone that has a copy of that book. I'm guessing since they spend so much time talking about solar power farming that the actual power requirements for the fusion plants are much lower than expected even with the growth of technology and urban sprawls in the setting. Also it's possible that the Blue Sun reactors are much more efficient than the numbers cited above. Maybe a better metric to follow is if we have around the clock shipments of He-3 going down the Beanstalk for 40 years already then does that math make sense? How heavy a load can each beanpod carry do you think?

Add: Just to hammer it home OP the Worlds of Android book says 1000s of years so not in these character's lifetimes, which only makes sense given the scale of the project Weyland undertook to benefit from the He-3 import business. If the Lunar mining operation is going to extinguish in a couple years I doubt they would have bothered with the Beanstalk project and long term Lunar colony. The whole explanation of the Jack Weyland plan sort of assumes a "limitless" fuel source. If you want to make it a narrative plot point then give the Lunar mining operation another 20-30 years worth of resources. Then explain that this was just a stop gap to the gas giant mining that is currently being developed. That's probably plausible since they'd still need all that infrastructure anyway and decades of profit would probably make it a good investment. Also that would give the whole Lunar operation 70 years of profitability. Once a project extends beyond one life time it starts to feel like forever. People who started the project will have retired and/or died before they see it need to be replaced.