If you use PVC straps (designed to allow movement and required by 352.30) and run the pipe so that it has at least two 90 degree bends at the ends (go up, go long, go down), then the PVC will expand and contract along the long run, spreading the 90s. No expansion fitting is necessary. Article 352.44 allows this in the last part of the sentence: "...in a straight run between securely mounted items...".
I would leave the risers at least 2 feet long, but remember the first strap must be at no more than 3 feet from the box or LB. I learned this from watching the pipefitters make a long run of stainless welded pipe. They would make a series of turns every 200 feet or so to compensate for the expansion and contraction due to seasons, sun and weather.
Table 352.44(A) shows the expansion of PVC conduit.
