(I picked Parthia because AFAIK, in terms of travel time, it is at the extreme limit of the ancient Jewish diaspora. But I'm interested in answers for diaspora Jews in general as well!)
ORBIS: The Stanford Geospatial Network Model of the Roman World gives ~28 days as the travel time from Singara (not quite Parthia!) to Jerusalem in the Spring (Passover + Shavuot) or Fall (Sukkot). Assuming I plan to stay in Jerusalem for the 50 days between Passover and Shavuot, and make a separate trip for Sukkot, that's 28+50+28+28+28=162 days of travel/residence in Jerusalem per year. Wow!
Does this math check out? Is there something I'm missing either in contemporary travel realities or in contemporary Jewish halakha that would make this amount of travel time unnecessary?
My assumption based on this figure is that most Parthian Jews would not find it feasible to make these pilgrimages yearly. Are there loopholes/alternatives for me if I want to be a good Jew but simply can't make this thrice-yearly pilgrimage work in terms of my financial constraints? Can my community send just one pilgrim every year to make offerings on all our behalf? [edit to add: Do we know if these pilgrimages ever/often doubled as mercantile trips?]
Thanks for your helpful answers!
As a follow up question would the pilgrimage have been an event in itself i.e. would people have gone to visit various sites along the way or would special events and entertainment be put on in different areas aimed at the people passing through?