What would be the best way to allocate the 90 minutes to maintain my lifts, in terms of volume and intensity?
Maintaining mass and strength requires a lot less training than increasing does. A 2011 study which Bret Contreras wrote about found that as little as one set/week (taken to or close to failure) can be enough for maintenance (of thigh lean mass and leg extension strength, for 20-35 year-olds).
I wouldn't bet that a single set is enough for "the Big Three" squat/bench/deadlift though, but in 90 minutes you should be able to squeeze in 6 sets of each (5 min/set, which is plenty, but you need some warmup sets, too). If these 18 sets are taken close to failure, that should be plenty to keep strength and muscle.
If you're also doing bodyweight exercises, I'm guessing you get a fair amount of arm/shoulder/chest/upper back training, so during your Big 3 workout there are a few reasons why you'd may want to dedicate a bit more time to squats than to bench/deadlift:
- Legs are usually the hardest part to train with bodyweight exercises, unless you're a hardcore masochist who loves doing stuff like Bulgarian split squats to failure.
- The bench press muscles (pecs, triceps, anterior delts) are worked pretty well by dips, pushups and other exercises which I assume are part of your bodyweight training. No, they don't work the muscles exactly like how bench does it, but it's fairly close, so you don't have to worry that much about volume, as long as you get a couple of good sets of bench every week.
- Both squats and deadlifts work your lower back pretty good, but squat strength carries over to deadlift strength better than vice versa, so doing lots of squats will benefit your deadlifts more than your squats would benefit from doing lots of deadlifts.
A final idea is replacing some (or all) of your regular deadlifts with Romanian or stiff-legged deadlifts. They hit your hamstrings better than regular deadlifts do (and your hamstrings won't be doing a lot of work otherwise unless you throw Nordic hamstring or some such into your bodyweight training). As an added bonus, you can use less weight for RDL/SLDL, so you decrease the risk of falling through the floor down to your neighbours.
So if we assume you have 18 sets to work with, something like this seems reasonable, assuming 2(±1) reps in reserve:
- 10 sets of squats (enjoy!). This is pretty good volume, as the sweet spot seems to be 10-20 sets/week/muscle group. Start off with a few heavy, low-rep sets (say 4-6 reps) to stay accustomed to heavy weights, and finish off with lower weights and more reps (say 10 reps) for volume.
- 4-5 sets of bench. Focus on heavy weights and low reps - your bodyweight training can provide volume for muscles involved.
- 3-4 sets of (Romanian, stiff-legged, and/or regular) deadlifts. Since you mention previous injuries, I'll magnanimously let you decide on reps and weights yourself.
Wait, did I say this sounds reasonable? Deadlifts after you've done 10 sets of squats sounds positively horrible - if I were you I'd do deadlifts and squats on separate days, but that's your call.