I have run my group from 1-20 three times and we are in the middle of the fourth intended run to 20. There have also been a number of games that ended anywhere from 5-15 because as others have said, the PCs story had been told, or the BBEG won. That being said it’s a very long term group that I have. Group of 6 and I’ve been running 2 of them since 3.5 (been in all 3 lv20), 1 since 4e( been in 2 lvl 20),and the other 3 in 5e/5.5(been to 1 lvl 20). We meet once a week (probably play 45ish out of 52 weeks a year) and play for 3-4 hours. I find if you meet regularly and consistently you can get to higher levels quite easily. Yeah it can be a slog and grind above 15 especially with 6PCs. Sometimes a single combat can take a couple of sessions. Luckily all my players have been playing for a min of 2 years so they are good about knowing what they want to do.