the mentioned 90 days is just a preview of your segment. that means you can see how big the segment is based on the data for the last 90 days.
however, the segment is NOT restricted to this timeframe by default. that means when you apply the segment anywhere, it takes the timeframe from surrounding settings (eg workspace reporting timeframe).
as far as i know you can‘t change the preview timeframe, but i can‘t see any advantages if you could ...