A topic near and dear to my heart.
USERRA protects your civilian employment position for a cumulative 5 years per employer. The 5 year counter only applies to long tour mil leave, which is defined as absences of 31 days or longer. This is regardless of whether the duty is voluntary or involuntary. Some duty, like involunatry deployments in support CONOPS (contingency operations) are exempt from the counter, meaning they do no count against your 5 year limit. Formal schools like basic training for instance, are exempt from the 5 year counter.
Now the reality. I hate to say it my man, but civilian employers are not as a patriotic as they pretend they are. I say this as a career through and through AF reservist (full time). There was a time where I toyed with the idea of holding a civilian job while as a TR (traditional reservist) and what I quickly found out is that civilian employers support your service...as long as it fall outside the hours of M-F 9-5. IOW, their support and yellow ribbons, and a buck twenty gets me a cup of coffee. So I decided to pursue the Reserves full time and haven't looked back. Took me a while to secure a full time position through the Lost Decade, but that's a story outside the scope of this thread.
I recognize non-aircrew Reserve life is much closer to the one weekend a month, 2 weeks a year trope they advertise on TV, and perhaps that may be more palatable to your employer, but unless your employer is the airlines (as a pilot) or the federal government, I wouldn't touch the Reserves to be honest. You will be penalized in civilian life in the form of lack of promotion and/or outright harranguing over particcipation, and you'll never be able to prove this discrimination in court without a long drawn out effort. For me it wasn't worth it, but I was a pilot and could make more as an activated reservist than a civilian non-tenured college professor, and with better QOL than the Active Duty component I wasn't willing to join for said QOL reasons. I recognize my situation is a specific one-off and doesn't truly track the lifestyle of a non-aircrew pedestrian Reservist, but I wouldn't do it.
Individual involuntary mobilizations are all the rage these days. It is a very difficult thorn to negotiate, as that is not the way the Reserves were built for originally, and it absolutely destroys retention. You'll never get that skinny from a recruiter, and I would go so far as saying they are the absolutely worst source to ask that question to. You need insider information such as what we're providing here for you.
Now, if you're gonna utilize the Reserves to get a new skillset, that you then can flip into a more lucrative civilian job and then quit the reserves when it becomes a hindrance to that new civilian job, I wholesale support that. you'd be doing more than 99% of Americans do in service to their country, and getting a leg up on a betterment of your family's situation. But I wouldn't do part time reserves as a non aircrew for 20 years at the expense of my civilian job, outside of the employers I listed before. No way. Sorry man, but that's my opinion as an insider. Feel free to PM me if you want to discuss more specifics about your situation.
Envy