I have run the Corellian Shuffle and Debts to Pay - both include a timing mechanic of some sort.

Tomorrow my players will sneak into a Black Sun hideout and attempt to rescue a friend. There are portions I'd like to have timed.

How many rounds are too many or too few? I was thinking of giving them 10 rounds at the time the slicer turns off the security system to try to get their friend and get out before the alarm goes on. Does this make sense? I want it to be tense and don't want to give them too many rounds, but I don't want it to be too few either.

The way I'd approach it is have triggering events and time delays off of them. If the party manages to evade detection, there really shouldn't be any kind of a time limit (unless it's one imposed by an outside party -- say So-and-so is visiting at Y time, so they have to be gone by then).

I'd also have various stages of "response" to the event. Immediate (1-3 rounds), intermediate (5-7 rounds), sluggish (10-12 rounds) and delayed (>15 rounds). So if they trip an alarm, an immediate response might be to have a squad of troops show up in 3 rounds. If they cause an unexpected power surge, they might send some techs to take a look at it in 7 rounds. If someone manages to redirect those techs by inputting some conflicting orders, they might be delayed and arrive in 15 rounds.

Sure, you can set a number of rounds as a limit. Do you have a rough plan for the sort of things they have to do in order to reach their friend (number of doors to break open, that sort of ting)? One way to give your players a little control over the situation (and increase tension at the same time) is to set how long any given task takes and then have successes and failures modify that time. For example, they need to get through a locked door to enter the "prison area". Normally this would take 2 rounds out of their precious 10, but if they manage the Skulduggery check with 3+ uncancelled successes they cut it down to a single round. If they fail the check, on the other hand, they still get the door open but now it takes 3 rounds instead.

Throw in a few situations like that using different skills, and pretty soon everyone at the table will be hanging on every skill check made. Plus, it gives different players a chance to shine; Skulduggery to open doors, Stealth to sneak past some guards instead of having to take the long way around (or shoot them, which would take precious time), Perception to keep track of which direction you're going in and not get lost, and so on.

Play through it yourself once or twice using their characters to see how long it can take.

I have had a lot of fun with using a real timer. PCs need to disarm a bomb put a egg timer for 15 mins on the table. gives the PCs a real sense of urgency, cuts down on meta game. but the work for you is to know your encounter front to back. this way you don't waste the PCs time on your actins. it can be a bit chaotic but it, is a ton of fun.

Most timers have a way to pause them, so while you’re looking something up you can hit the pause button and not spend their time.

I've had luck using accumulating successes as a timer in combination with rounds - that way a really good roll clearly helps the group.

One thing I've always wanted to try but have never gotten around to is to have some background music that increases in tempo gradually each round. Maybe it gets a little more dissonant as well.

I hesitate to use a physical timer at my table because, well, we're beer league roleplayers. However, the incessant ticking would certainly set the mood!

I usually reserve "rounds" for combat. If there's a time sensitive mission, I use real units of time, like minutes or hours. I then have some ranges in mind for the end result. If they do it less than X they did great. If they finish between X and Y, not great, but not terrible. If they take longer than Y, uh oh.

The results of checks along the way add to or subtract from how long that task takes them.

If there's going to be combat along the way, assign a certain amount of minutes to each combat round.

For your scenario, I'd probably do something like 20 "minutes" from the time the slicer gets them in and when the alarm goes off. Checks along the way to avoid detection (stealth) and figure out where the prisoner they need to rescue is (computers or some other source of this info). Successes and advantages mean they get it done quickly. Failures and threat mean it takes longer (and maybe they run into a patrol and have to talk their way out of it or fight). Each combat round is a minute each.

There's a lot of book keeping on the GM's part for this sort of thing, but it certainly increases the tension when the players know they're working against a real clock.