You don't have to do all of your time all in one swoop. Depending on the unit and travel costs, you can break it up. There is an IMA guidebook somewhere with some more details, but they will usually only pay travel ONE time for your use of annual tour. And of course, just as a Traditional Reservist, you are on your own for any IDT training travel costs.
So, just like a TR, you have the equivalent of one weekend per month, or 24 days (48 periods) per year to burn plus your annual tour of 12 days (this is for Category A IMA, Category B is 12 days/24 periods). So, in short, you are on the hook for 36 days per year, which can be done all at once or broken up, depending on the unit and your supervisor/scheduler. The big catch is that if you break it up into more little chunks, you will wind up paying more for transportation out of pocket if distance is an issue. Also, if you do all of your days in one chunk, you better be damn sure you have done all of your annual requirements while there (PT test, physical, training, CBTs, etc), because you will be SOL if you have to take a PT test 6 months down the road and you don't have any IDT or AT days left.
And of course, since IMA jobs are usually tied to active duty units, there is usually no such thing as "drill weekends," which means most of your days will be done Mon-Fri. Most supervisors will let you burn a single period or two over the weekend for admin/CBT stuff, just to keep your lodging paid for by the IMA detachment. But a week of working your IMA job will likely only be 5 1/2 days of pay as opposed to 7 days of pay. Unless of course, you are in some exercise or in an office that works 7 days per week, then ignore my last sentence.
IMAs, just like TRs, can whore themselves out and take on additional maydays in the form or RPA, MPA, or whatever days. Obviously you will deconflict with your IMA unit if you are going to take a longer MPA/RPA tour, as a lot of IMA jobs want you there for a specific exercise or time frame in order to maximize your usefulness and experience.
nunya, I think I answered all of your questions, but just to summarize: no IMA jobs are really paid "full time" for any months. At best, you are paid "mostly full time" for 36 days if you shoot your entire wad of military days in one fell swoop. Of course, if you did an additional volunteer active duty tour, then you would be paid active duty for that amount of time. IMAs are typically not weekend warriors, TRs are the weekend warriors (unless you are like I was and did drill makeups during the week in order to preserve your weekends). Both IMAs and TRs are Reservists, and both are on the hook for 48 periods, plus Annual Tour each fiscal year in order to get a "good year."
You will want to make all requests for IDT and active duty days well in advance - at least a month. And most people complain that working through the Detachment as opposed to their own unit orderly room can be a pain. This article might be worth a read:
https://www.theboohers.org/my-start-as-an-ima-in-the-air-force/