Answering any questions for the Reserves is a bit tricky, because (at least when I was in) so much was dependent on the unit. My unit was essentially for backfill - unless the US was invaded by Canada, we weren't going anywhere as a Combat Support Hospital. Our senior NCOs and physicians got involuntarily deployed to Iraq and Afghanistan; all the registered nurses that went volunteered to go. On the other hand, there are reserve units that can be deployed as a unit - in my time, there was one connected with the 101st that got sent out more than once. They were higher up the food chain in terms of available training for their soldiers. As k2rider said - it's 4.5 years before Active Duty got a school - and you can count on the Reserves being even longer than that. It wasn't until after 9/11 that our unit finally got rid of the last of the Vietnam equipment. All of our designated ICU nurses got that credential in the civilian world, not from the Army - except for one or two that had been active duty first.
A couple of comments on Active Duty vs. Reserve for military nursing (although you didn't ask). The military starts you out at lower pay than the local hospital - but in 8 years you'll be out-earning your civilian peers, with the prospect of more responsibility and further increase in remuneration. The military nursing model is one of leadership - you are expected to guide and direct the enlisted soldiers (and later, junior officers) under your command to provide the best care possible for your patients. The civilian model is a lot muddier. Nurses have very little real authority over anyone other than themselves, although they are expected to be totally responsible for patient outcomes. You have no authority to make the pharmacy tech bring you your newly ordered timed medication before he takes his lunch break, for instance. You are the eyes and ears of the physician - if s/he doesn't order the right thing, it is your fault because your communication to the provider wasn't clear in describing the patient's condition. In the civilian world, you will NEVER outrank a physician, and in my experience, since nurses are easier to come by than physicians, hospital administration will rarely back you. Not a big deal as a new nurse, but rather frustrating when you are dealing with a terrified new resident on July 2nd who is unwilling to acknowledge 20 years of field experience. In 30 years of nursing, that has been my constant struggle - finding employment where my responsibility is matched by my authority. I've been away from the bedside for a few years, but listening to anecdotes from friends that still are, not much has changed.
Also, I live in flyover country (Michigan) where new bedside nurses are expected to work every other weekend. With your Reserve obligation, that means you have a grand total of 1 weekend per month to actually pursue a social life when everyone else does. This may not be a big deal to you, but is something to keep in mind. And when it is 'your weekend' to work your civilian job and you have drill that weekend, expect your civilian managers to not understand that you are not asking for vacation/PTO time. Most will encourage you to 'find someone to trade with'. If you are working in a smallish hospital, this problem can be magnified.
Good luck, and let me know if there are any other questions I can answer.