I think TC wrote an article called 3,2,1 or something like that…3 sets of (eg banch press) 2 sets (eg dips) 1 set (eg flys) and that’s it per body part.
Tried this approach when I could only train during lunch breaks and had to eat and shower too - worked preety well and workouts came in at sub 20 minutes…
Now that I think about it, it was a method for prioratising a weaker body part i.e. putting emphisis on the weaker delt head (3 sets) while doing little for the dominant one (1 set)
HIT training can work too - but I know they guys on here don’t like it so much!!! 20 rep squats and deadlifts will sort out the lower body pretty good and Bench, rows, military and chins will do the same for the upper body if you work intensly enough.
If you only want to maintain, Hans Seyle states that frequency can be like every 7-10 days and you won’t experience regression.
Another approach might be Super slow (hang on…don’t flame me yet!!!) which whilst it sucks for hypertrophy and strength, as a general fitness training modality, it might fit your needs.
You could always prioritise your time so you have more for the important stuff like training though!!! 