I've found Stonehell a little treasure-light. I did a quick count of gold on the first level. There's about 22,000 gp worth of treasure. Labyrinth Lord doesn't give XP for magic items. With XP for killing monsters, we'll call it around 25,000 available experience points.
(Also, treasure in Stonehell is distributed unevenly. Most treasures in a quadrant are very small, but one or two are large. Further, there's a great disparity of wealth between quadrants. The least lucrative quadrant has less than 1,800 gp. The most lucrative has 10,200+ gp. This is in line with the megadungeon design precept that well-traveled areas have already been looted, but it may not be obvious from a player's perspective.)
The average number of experience points necessary to reach second level in Labyrinth Lord is 2,234.
This will vary a bit from group to group, but figure that a quarter of first level PC's will die and the players will miss or bypass a quarter of treasures. Assuming players explore a very large chunk of Stonehell's 140+ first level rooms, there's enough experience for 5-6 characters to reach second level. And, assuming your group clears 6-8 rooms per session, it will take them about 20 weeks to get there.
Stonehell has been touted as the first published campaign megadungeon—something on a scale Gary's players might find familiar. Men & Magic says,
"from four to fifty players can be handled in any single campaign, but the referee to player ratio should be about 1:20 or thereabouts." Those twenty players would find Stonehell a hardscrabble place.