It depends. Line might clear out a little, might clear out a lot, but no way to predict. We have more experience at MK riding BTMR during parades/fireworks. Posted wait time 30+ minutes would actually be less than 5 minutes during parade, but as soon as the parade ended the line would get swamped, posted 15 minutes but would really be 45. Since a lot of the RnRC line is outside, you can generally tell if the posted wait time is accurate. If the FP return line is backed up outside, you know standby will be long and slow.
Another option for RnRC would be single-rider line. We used that extensively when last there because of the FP+ tiers. The line once you got into the parking garage was typically between 5 and 15 minutes, depending on length and how good the CMs were in filling trains. One CM was falling behind and would call out "3 single riders" so my wife and I ended up riding together. Other CMs would try to match up odd-numbered parties and the line would crawl.