$$$O(1)$$$

To specify, I assume this is C++. For a RandomAccessIterator (for e.g. a vector) this is indeed $$$O(1)$$$. If it's a weaker form of iterator, e.g. a BidirectionalIterator from a set, then first of all it won't compile, you will need int pos=distance(it1, it2) and also it will be $$$O(it_2-it_1)=O(N)$$$.

If you are more innterested, you can have a look at the specifications of iterators here. I think the two iterator types I named are the most common ones in CP (just my feeling).