A milestone without a time limit is a to-do item. It can sit undone forever and nothing flags it. The limit is what moves an account from slow to on hold, so it is worth setting with some care.
Start from accounts that went well
Take your last ten accounts that finished onboarding without trouble. For each milestone, note how many days it took from the previous step. The limit should sit a little above what those accounts needed. If most of them connected their data within three days, a limit of five gives normal accounts room and still catches the stuck ones.
Do not set limits from what you wish would happen. A limit tighter than your healthy accounts manage puts good accounts on hold, and a hold list full of healthy accounts teaches the team to ignore it.
Different steps, different limits
Steps that depend only on the customer, like inviting their team, can have short limits. Steps that wait on someone else, like an IT approval before a data connection, need longer ones. If one step routinely waits on a third party, say so in the milestone name so the owner knows who to chase.
Count from the previous step, not from signup
Measure each limit from when the step before it was done. Counting everything from signup makes a late first step push every later step on hold at once, which hides where the delay actually started.
Revisit after a month
After a month of real accounts, look at the hold list history. If a step is on hold for nearly every account, the limit is too tight or the step is really two steps. If a step is never on hold, the limit may be too loose to be useful. Adjust, and write down why, so the next person to change it knows the reasoning.
The onboarding checklist guide covers the rest of the process, from describing a working account to running the hold list each morning.