The experience will fully depend on the office you're working in and the sub/RCA staffing levels there. For me, we had 11 rural routes across 3 offices in the city. There were times that we had only 2-3 RCAs to fill in for the regular carriers. With regulars out for surgery or on PTO, it meant that I was literally working every day (because of Amazon on Sundays). The scheduling for RCAs is based solely on seniority, and even if you're "off" expect to be called in the day of to work.
On the flip side, sometimes we'd get RCAs to come in from other offices in different areas because they didn't have enough work. One gal that helped out said she'd only had 4 days of work for the entire month. She needed more work and was trying to transfer to our office, but her PM wouldn't let her leave. So, expect a lot of bureaucracy as well, it is a government job after all.
Because you're only guaranteed one day of work each week, this position is technically considered part time. And because of this, your benefits are minimal. Even if you're working 70-80 hours per week during the holiday months (and you will), you're still only eligible for heath care contributions. I wish RCA benefits were something the union would focus more on negotiating to help with retention. PTO is only able to be accrued if you're lucky enough to be able to hold down a regular route for a certain period of time (i.e. regular is out for surgery or actively works for the union a lot) or have enough seniority to get a regular auxiliary route if your office has those.
The length of time to go regular will also depend entirely on your office, how close to retirement people are, and how many RCAs are ahead of you in seniority. My office had 3 regulars that were ready to retire in the next 3-4 years, which is relatively short. I've heard of small offices with only a handful of routes that wouldn't have any regulars retire for 10-12 years. So, it's really a question of whether or not you're willing to stick it out as an RCA for a long time.