Dur to my duties it's hard for me to sit for more than 30 minutes straight on a regular basis. Ideally I'd like to find the time to sit at least 30 min twice a day, but it wouldn't be regular enough to build some momentum.

So I was wondering if multiple little sits (~15 min) throughout the day, let's say every 2 or 3 hours, could be a solution or if it not as good as 1 or 2 long daily sits.

I have not enough experience to conclude by myself, do you advanced practitioners have a view to share on this?

I've found that multiple sits of 20 minutes can be effective, if kept up regularly. However 15 minutes is generally too short a time for the mind to settle. That's just my experience, of course. Yours may be different.

Alan Wallace recomends multiple 24 sessions a day, even in retreat, if you're not advanced. In a recent dream yoga retreat, we would do two sessions with 2-minutes intervals, then another session after the dharma talk, in the morning and in the afternoon. and another session before bed. so seven 24 minute sessions a day. very good. but, I would be guessing, specially for concentration, you will ocasionally make some longer sessions, even if not daily (maybe on retreats?), but I presume, reading the progressions on concentration, that they should not be very hard to do when you're ready for them (Lama Alan says concentration gets easier with progression towards state of shamatha, where you can stay almost efortlessly on object for at least four hours).

I agree with this.   This past summer I did that with the intent of making my sittings more shallow.

My last parent died at about the same time an LTR ended.   I had too much to deal with at one time.   I broke my sits up into multiple 20 minute sits with breaks in between to give my body a chance to relax and keep my practice up without relaxing enough to let all sorts of disturbing stuff flood my mind.