In my experience, it takes around 35-40 sessions to reach level 8. I'm a big proponent of systems where you actually get to reach the later levels and do cool stuff, so this feels like a good amount of sessions before players get to retire their character and wrap up a campaign. The first few levels go fairly quick (players will generally reach level 3 after around 3 sessions) but it slows down a bit after that.
As for adventures to sustain a campaign of that size, issue 02 will have two adventures in it which will roughly take a party up to level 3 or 4, and has a system for handling downtime in between. A bigger third adventure is also planned for after which will probably take players up to level 6. But TEA is made to support sandbox play, so just letting players come up with their own goals and letting them run wild using only the factions and encounters in the first issue will get you pretty far if you're comfortable with some improvisation :)