I think 2-3 weeks per level is perfectly normal, especially if, like me, you only do reviews once per day. 96 days on a level is definitely quite high, though. I think you may simply have too many unfamiliar items in your queue affecting accuracy and level-up times. Counterintuitively, you may want to slow down on lessons to speed up your level-up times (my Ganbarometer script linked below can help you manage your workload).
Here are my level-up times for comparison (note that things started getting harder for me around level 18 as well, though my worst was level 26 at 38 days):
The GanBarometer userscript will show all the details of how many reviews per session and sessions per day you’ve been doing for the past few days if you’re interested.
The Heatmap userscript will also show the total number of sessions (using a simpler algorithm to determine what’s a “session”).