Time:
I like the 15-30 minute routines the best because if your smart about it, you can do the routine everyday, make progressive gains, and not be sore all the time, but you will not ‘bulk’ up quickly, you will gain strength, and get bigger, but size will come later doing it this way.
Also I should mention that I switch up my training all the time, so i might do this so called interval for several weeks, then change to something entirely different - im not really interested in gaining size, unless it happens as a consequence witch is okay by me.
If you can make an hour’s worth of time, then you can go with a more ‘traditional’ approach to gaining size. Bench and squat and stretch. If all you have is dumbells, you might have a hard time doing weighted squats, do bodyweight squats or even hindu squats (different technique).
Pushups and pullups will help with size, go to elitefitnesssystems.com (i think) - they have an excellent links section and articles section, do your research on sets and reps and size gains.
The main thing is going to be figureing out what you CAN do, as in what movements/exercises/equipment you have to utilize, then making yourself a training plan/routine, and stick to it, take measurements, and make adjustments if necessary.
I have what I call 2 “outside” routines, and 2 “inside” routines. The outside ones are for working out - out doors, basically without any equipment, the inside ones are for indoors, with equipment. For outside and inside, 1 routine is a ‘interval’ routine for 20 minutes, the second is a ‘rep’ routine, where I do the same number of reps for each movement, with no rest, never working the same muscle consequtively.
Maybe Im getting long and complicated now, specially considering that you are looking to bulk, im just looking for functional strength.