Longer Periods of Downtime
Events
PCs who want to do things that don't correspond to a specific downtime activity should still experience downtime events; you just choose the relevant skill and DC. For example, if a character intends to build a computer, you might decide constructing the machine and setting system preferences once construction is finished are major events. The first could be a Crafting check, and the second a Computers check.
Average Progress
The events you include during a long stretch of downtime should typically feature higher-level tasks than the baseline. For instance, a character Earning Income with Piloting for 4 months might work at a port doing 1st-level tasks most of the time, but have 1 week of 3rd-level tasks to account for busy periods. You'll normally have the player roll once for the time they spent at 1st-level tasks and once for the week of 3rd-level tasks.