Considering our wealth, I'd say before the next turn is over, probably.
Excellent. Soon that DM will stop complaining about my nice rooms and I will be able to make sensible demands! BWAHAHAHAHAHA!!!
Ahem.
Oh, so that was changed?
Well, in order to make a pit that collects bolts, you need to make a longish archery range with holes on the sides of the archery targets, so that misses that go off to the side will go down the holes. (The longer the range, the more chance of the bolts going astray.) Putting holes in front of an archery target breaks the "room" designation, and prevents the whole room from being used. You also have to designate each archery target as its own archery range.
I tried doing that once in 40d, but it was such a pain collecting individual bolts that I got sick of it, and just patched up the floors and didn't ever try it again.
How about designating the room but NOT telling squads to train in it yet, channel in front of the targets, and THEN telling the squad to train?
Or putting floor hatches/a bridge in front of it, above the pit, and opening/retracting them/it with a lever before use?