DCA-LGA now is seven flights weekdays, with large gaps (3-ish hours) in the afternoon.

DCA-BOS is nine flights weekdays, with more consistent frequency as a result of the x9.

I remember when the DCA-LGA Shuttle was hourly pre-Covid. If you missed the wait to the next flight was not long. I assumed (incorrectly) that Shuttle was only the most frequent service, not boarding cutoff time.

I was recently on the 2:40pm DCA LGA flight, which was a full flight, sold out a few days prior to my flights (I checked this to see if volunteers could be needed). Thr next flight after 2:40… 5:59pm.