I don't keep track of how long I read, because my mind often wanders, sometimes I go do something for a bit and then completely forget I was reading and end up doing other things. Instead I keep track of the dates I start and finish VNs. Obviously that isn't a good way to measure their lengths, but it's interesting info.
Also what you can do is see how far you can get in, say, 30 minutes of reading, and then restart and see how long it takes you to skip that far.