It varies wildly. There are a lot of factors that go into it.
How big is the adventure?
I've been in games where an entire story went from level 1 nobody to level 20 world saving hero in about 40 sessions of 4 hours each, played weekly. I'm currently running a game that's going to take two years, with 3.5 hour sessions weekly (we're over a year into it, and that time estimate is doing well).
Before that I was in a game that was following a set of published adventures. That game was going before I started, and will be going after I finish. It also meets weekly, but they have more side quests and other things going on.
There's no real guideline for how long a storyline should be, except that you want something your players can finish. If you only have until the end of the semester before people go back to university or something like that, you can't run a 3000 page adventure.
How fast are the players?
Some groups operate quickly. Everyone knows what to do and they'll keep combat moving at a high speed. They may also be focused on the current goal and ignore other things (or they may not).
Other groups won't. Sometimes you get newbies who need more time to figure out how to react. Sometimes you get people interested in exploring the world and they'll go off on side quests, or random tangents not related to anything because they want to see what's around the corner.
And sometimes, you get groups that for one reason or another wind up doing a lot of out of character talking during games. That also slows things down, as the game isn't progressing while people are telling jokes or catching up. And you know what? If everyone is having fun doing that, great! I suspect you might be having this happen somewhat, as 7 hours is a really long time to stay in character for without breaks. There's nothing wrong with that, but a lot of it just means things will take longer.
How descriptive is the DM?
Some DMs will be description light, and that makes things go faster than DMs that are description heavy. For players that want a fast paced game, being light on description can be really helpful to keep things moving.
I had a DM once who was very detail oriented. One session we made offhand comments about solving our money problems by robbing a bank. Next week, he came to us with a detailed overview of how the banking system operated and the bank security, should we want to do that. He spent 45 minutes going over that with us, based on an offhand comment. It brought the game to a dead stop, and it was awesome.
How much combat is there?
Combat can be slow, given the setup involved, and the actual dice rolling and such. If you have a lot of combat, you'll take longer to accomplish goals than if you have less. Random encounters can be a big one here.
I found that random encounters as people wandered around a dungeon were slowing them down too much from reaching the scripted encounters in the various rooms, so I just removed 90% of the random encounter rolls from the dungeon I'm doing. That let the players move around the dungeon more quickly, and made them happy.
In general, most of this comes down to time efficiency. There's lots of ways to get more or less done in the same time at the table. Beyond those, it's really about the scope of the storyline and the play style of the people at the table. There's no real hard numbers or guidelines for it, but in my experience yours is on the longer side.
From my own time as a player and DM, here's some numbers I've seen:
- My current campaign - Currently at 58 sessions, 3.5 hours a session. 203 hours. They've completed 5 dungeon areas which have their own story, along with some occasional side quests. So we're in the 30-40 hours per story range, but these are all one story arc. Right now I estimate 80-100 sessions to complete the entire story.
- Last Campaign I played in - This one took 40 sessions of 4 hours each, so 160 hours. The story had 3 parts, but again it was all one major story. ~54 hours per story part.
- One Before That - This one was a series of published D&D aventurers. I believe they had estimated times on them, but we were underpowered for what we were fighting (due to the DM giving well below standard loot, class selection, and generally being a party that wasn't optimized at all). The result was that we had to be very cautious or die. Combat and exploration took a VERY long time in this group. It has to be over 100 sessions right now and still going, and I'd put each story piece anywhere from 40-70 hours depending on the specific one. (We literally spent over a month trying to clear out one floor of a tower.)
Those numbers won't hold true for other groups, but they give you an indication of just how wildly things can vary.