The following is from the IWD wiki fan site:
"Experience cap
There is no direct experience cap in Icewind Dale; however, there is a level cap of 30.

This means that the maximum amount of experience that a character can benefit from, depends on their class: For example, a single-class Thief reaches level 30 at experience 4,400,000, while a triple-class Fighter/Mage/Cleric reaches level 30/30/30 at experience 22,500,000."

This said there is a limited amount of experience available from quests. There are a few areas where you can "farm" experience. I.E., monsters will re-spawn once you leave that area and then come back. So "technically" there is no limit to experience points available. However, doing the math, it takes a lot of extra trips to gain another level this way.

But depending on each class of your six members they will reach differing levels at the conclusion of each of the three adventures. Also other than splitting the experience 6 ways your party composition will play a factor if you have any multi-class characters. Multi-class will advance more slowly as they have to split their "share" of experience between the classes they have (usually 2). The difficulty level you play at will also affect how much experience you will earn. You can view the differences in the "options" page by using the difficulty level option. The difference affects both monster kill and quests.

"My" typical run throughs with a 6 member party yields by the end the original game a level of 14 to 17 (depending on class). HOW was (IIRC) about 18 to 21, and TotL about 23 to 26.

There are points in the game where you can transition to the other adventures without completing the one you started. For story continuity some choose to do this rather than start a fresh game (some items will not carry over to a fresh start but do with a transition).

Sorry for the wall of text. : )

The party leader in my 6-man Insane playthrough earned 2.79 million XP by the end of IWD w/o HOW, increased to 6.36 million XP after finishing standalone HOW (w/TOTL).