Hello shade;
In one project we did 25 temperature loops in a S7-313C-2DP processor, all in OB35, set at 100ms. Of course the rest of the program was as compact as we could make it (we used the available technological functions to control heating SCRs), and we used multi-instance DBs to limit the amount of memory we needed to process the whole project. But the OB35 call was not a problem.
Your main limitation will be the work memory available on your CPU, not the cyclic OBs.
Hope this helps,
Daniel Chartier