I don’t want to put my future coworker through six rounds of interviews. If it takes more than three rounds + a phone screen to figure out if someone is a good fit then the process is broken.
Whether it's reasonable depends on the distribution, not just the duration.
A 2 hour onsite with the candidate being rapid-fire interviewed by six different different teams and a 20 minute call every couple weeks for three months are very different (and select for very different types of candidates) despite having the same overall duration.
If you think that’s unreasonable, please go ahead and add a few fire sauce packets to the bag for me.