Is it normal for a molar to be loose after rct? also how long should one wait before getting the crown after rct?

An abscessed: Infected tooth can demonstrate mobility. Either an endodontic lesion or a combination periodontal & endodontic lesion. Best to have definitive restoration (crown) on hold until the tooth demonstrates no further symptoms - no pain, no tenderness to percussion, no mobility. Sometime the tooth may have a poor prognosis to helpless prognosis even despite rct therapy & may still need to be removed.

Root Canal treatment: If the tooth had an infection prior to the root canal treatment, then the bone and supporting tissues surrounding the tooth may have also been infected. This will take months to heal completly, causing tooth to be a little loose. Crown should be done right after root canal treatment to ensure no bacterial leakage into pulpal area, and that the tooth doesnt crack, rendering it non-restorable.