Congress returns from its August recess on September 8 and faces a busy 9-day period in which to act on the Iran nuclear deal. Under PL114-17 Congress has until Sept. 17 to act on the Iran nuclear deal.
It can consider a joint resolution of approval, which may be offered by Republican leadership to show the limited support for the agreement. Such a measure could be filibustered in the Senate unless 60 votes are obtained to cut off debate and force a vote. I'd expect the House to schedule a vote to embarrass the administration. In the Senate, it's unlikely unless the GOP is confident of having more than 60 votes. [It's awkward to explain a vote to end debate on approving the deal.]
Congress is very likely to consider a joint resolution of disapproval, which could also be filibustered in the Senate unless 60 votes are obtained for cloture. If the joint resolution is passed, it is subject to a presidential veto, which can be overridden only by 2/3 vote in each chamber. In the Senate, the veto override question can also be filibustered.
If no measure is passed by both houses, presidential action lifting sanctions may occur starting Sept. 18.
If a resolution of disapproval passes but is vetoed, the law forbids any sanctions relief for another 12 days, to allow for a veto message, and then another 10 days for action on a veto override.
Any time thereafter, Congress may take up new legislation re-imposing sanctions under rules limiting debate to 10 hours in the Senate and two hours in the House. The law provides special treatment only for “qualifying legislation,” defined as a bill “reinstating statutory sanctions imposed with respect to Iran.”
It's interesting that the Republican leadership agreed to the law last May, knowing that it made things easier for the administration and harder for opponents of the expected agreement. Maybe they wanted the political issue more than the policy outcome.