M101

It's hard to say. For bright objects, like Orion or Andromeda, 60 is definitely the upper limit. For fainter ones, like this, it looks optimal.

The issue isn't overexposure of bright objects, but rather that under such conditions the sky background will be very high, causing faint objects to be lost.

Makes sense, I'll have to experiment with it.

If you use NINA, you can immediately determine the maximum exposure time for your conditions.

I checked the histogram of the raw frames: in the evening, the exposure was clearly too long, yes. But by midnight, it already looks reasonable—10–15% from the left edge.

The histogram doesn't tell you much. If you're comfortable with math, check out https://forums.sharpcap.co.uk/viewtopic.php?t=456. If not, here's a quick summary: you need to know: 1. The average bias for your camera settings, 2. The average background level for a given sub-exposure, 3. The camera read noise, 4. The conversion factor for your camera settings in e-/ADU. Recommended Exposure Time = 10 × (read noise)² / light pollution rate where light pollution rate is defined as: (median ADU of a subframe − median bias) × electrons per ADU / exposure length