It seems to differ based on routes. I have stood at wood quay at 7.30 at night waiting for a 39a. At that time of the evening the frequency would be every 20 minutes and buses would be quite full.
It would be quite common at that hour of the night to see 6 or 7 145s pass by at that hour of the night.
I've been on the 145 a number of times late enough at night and it can be standing room only as late as 10pm at night on a week night so the 155 with an all day everyday frequency of every 20 mins is a welcome boost to frequency and capacity on that stretch of corridor. Bearing in mind the 145 itself is only every 20 mins after 9pm.