Answer
Synchronization defaults to every 30 minutes, but that's adjustable — check the number of unsynchronized documents for a given Search Domain (from the 'Search Domains' window) to judge whether your data-change rate actually warrants more or less frequent syncing than the default. Rebuilding, on the other hand, isn't something to run routinely at all — it's only needed if the Search Domain's own definition changes or its index becomes unusable; scheduling a rebuild once a month 'for good measure' is a reasonable upper bound if you want one running periodically anyway. Keep in mind a rebuild locks the relevant tables for its duration (blocking business transactions on them), so schedule it for a low-usage window like nights or weekends — this applies to any Search Domain, not just Document Management ones.