The old NW guidelines for boarding was the following:

T-50 to T-60 for A330, 744
T-40 for 757, 753
T-30 for A319, A320, DC-9, E175, CR9
T-20 for Saab, CRJ

DL roughly still follows those guidelines.

My general rules are as follows:
-Assume T-40 for 757s
-Assume T-30 for 737, MD88, A319, A320, E175, CR9
-Assume T-20 for CRJ, ERJ, Saab

My actual time for leaving the SC and being at the gate depends on whether there is an aircraft there or not.
- If it is the first flight of the day, then assume boarding on schedule per above
- If waiting for inbound aircraft inside boarding window, be sure to be in gate area 5 minutes after arrival of inbound flight
- If Saab, ERJ, CRJ, CR9, who cares since its all planeside claimed, get there at T-15
- If no need for overhead bin space, who cares, get there at T-15