You can be IAP for up to 120 days, but effectively it's less since the RFMT/JO APPLY cycle runs quarterly. That cycle starts with a period of about a month when you can apply for jobs (there is a giant spreadsheet of all the billets), that part closes, and several weeks later the results get released. Then it starts over again. It's kinda like self-detailing. Some stuff is not so obvious- if you miss the window and your 120 days if IAP doesn't overlap to the next window, then you might be screwed.
Random thing that causes inordinate stress in the reserves: Don't forget to stick your CAC in an NMCI computer at least once every 60 days (NMCI webmail doesn't count... thanks, cyber policy geniuses). Say you miss one month's drill weekend (you can reschedule the drills, that's usually OK) but the prior month's and follow month's drill weekends happen to be scheduled >60 days apart. Better count days on the calendar... either make a special weekday trip to the NOSC before day 60 or call the IT person before that next drill weekend- that reactivation trouble ticket usually gets turned around the same day, but do you want to come to drill on Saturday morning and not have your card work until the next day? Oops, random coincidence but network maintenance on NSIPS just happened to be scheduled on Sunday... wish the card was working on Saturday. Heaven help you if you let that CAC go past 90 days...
The reserves are coming out with an app to do a lot of the online stuff- this includes a free card reader for your smart phone. Frankly, I think it's a brilliant idea and I hope it works as well as we hope. (The NOSCs have their own wifi networks, BTW, and they generally work well... with obvious limitations on privacy act stuff or getting something printed). Lots of millenials have smart phones but don't have a personal computer. Lots of reservists of all ages don't exactly have their laptop and card reader available 24/7. So here's how the app will be useful: most administrative stuff you do in the reserves has to get routed through a chopped by a chain, just like the regular Navy, but it's almost all web based. Email, evals, fitreps, travel orders, DTS, plus those random administrative distractions that were supposed to fade away.
Being cross-assigned is like having two bosses with all that it entails. If you're in your home NOSC OSU (most cross assigned people are, it exists for cross assigned people and for new people) then you can always help herd cats at your home NOSC; it will get noticed and it will be appreciated, but you will want to toot your horn to your real boss that you're pitching in instead of punching a clock on those weekends that he/she doesn't see you. Your real boss will see you only a few times a year when you travel there; it's human nature for people who get more face time to get ranked higher. When you do go there, they might do something like schedule Thu-Fri-Sat-Sun, which is good, but you'll probably have to take Wed-Sun off from your civilian job. If you have to be back to your regular job early on Monday morning then you're gonna be tired.
Random concept about cross assignment- you can be cross assigned to a like-unit, it doesn't have to be to the OSU at your local NOSC. That means you can drill with your local unit even when its billets are full. Seabees, for example, do this a lot. You can be a pilot in a nearby intel unit (such billets exist) halfway across the country but be cross assigned to a local intel unit, if such a unit exists. Another random concept- not all reserve units drill at their local NOSC (actually pretty obvious once you realize it).
Slight topic change- about people who failed out of mobilizations (IAs) at NMPS, especially for medical reasons. In nearly all cases there is a simple truth: the person decided not to go. Granted, the NMPS docs tend to dig a little deeper, and it's better to find a problem before deploying rather than during deployment. But before making it to NMPS, that person signed their name stating that they were ready to deploy and they looked people, in their chain of command, in the eye and told them they were ready. Then at the last second when they fell out at NMPS, someone else got tagged on short notice and another already-deployed someone else got extended. Keeping a secret or an honest mistake, odds are it was the former case. Think about that when you meet someone who failed out of a mob at NMPS, got tagged short notice and went, or got extended.