I'm GMing an once-a-month session with some friends. So far, I haven't killed anyone (yet) but they've gotten smacked around a few times. I know PCs are really tough and durable, so I ask the GM gurus this: how frequent is it that you kill a PC or make him burn a fate point?

Well, in our group someone has to burn every third or fourth session maybe. Sounds worse than it is, as every player has multiple characters to choose of, so there are plenty of fate points

Might also happen that someone has to burn twice in a session.

Edit: We like it grim dark!

It's "DEATH"watch, the clue is in the name. Nothing says whether it has to be player or NPC.

Nah in all eventuality unless the players actually do something stupid, the plot dictates/enforces it or the dice gods are just not in their favour then player deaths are actually not all that common. You have a good amount of HP to play with, decent armour and good dodges so that you can practically moonwalk your way out of any actual issue. Even then, fate points allow you to avoid being vaporised.

As a GM you might want to consider making them spend a few or put them in harms way. If they do not suffer some serious issues then arguably they will get complacent or think that everything is trivial. It's sometimes good to hurl something really nasty at them with the intent to bloody a few noses (or remove them) to keep them on their toes.

Our two year campaign ended up with only 1 burned fate point (rank 4 at the finish). We threaten a FP burn probably ever 3rd session. We were some of the luckiest S.O.B.s out there.

A memorable example was when a patriarch charged our tanky techmarine and took him from full to severed leg in one shot. Surrounded by genestealers, the gm started thinking that he had TPKed us. A few smoke grenades, lots of cohesion spent, and the blood rain perils saved the day. (the blood rain knocked the patriarch down from his reset ambush position allowing the devastator to put two concussion missiles into him.