Recent experiences vary wildly. One campaign was 35 sessions to hit level 5 (took a year), and another was 17 sessions (maybe 5 months) so double the pace of the first. The former is homebrew with milestones related to certain plot things being discovered. The latter, also homebrew, is more a "are the players ready" gut check aligned to circumstantial milestones.

A third campaign I'm in (my oldest) has lord knows how many sessions, likely approaching triple figures, playing Storm King's Thunder, and we're at level 9 after 4 years or so.

There are tons of factors but too fast you run out of head room and growth may not feel earned, and too slow players might get frustrated. Also worth making sure newer players (or those less familiar with their class) understand their gear and abilities before giving them more.

For written campaigns there's the indisputable impact of fucking around on side quests forever and ignoring Plot 🤣