Marriott doesn't have any limit on the number of points one can earn during a stay nor the number of nights for a stay. There are folks here who have very long-term stays (usually at an RI or similar extended stay property) of a year or more. Some things interesting things about extended stays:

1. As already mentioned, after 30 nights, it is possible that you might not have to pay as much in taxes as before. So it is worth checking on that.
2. Some hotels might still charge your CC weekly, bi-weekly or monthly (though Marriott will still consider this to be a single "stay".)
3. If you are staying at an extended stay property (RI, TPS, etc) you will earn fewer points per $ spent (i.e. 5 pts/$ instead of 10 pts/$)
4. Depending on your stay length, the hotel, etc, you might be able to negotiate a more favorable rate than what you might see using Marriott's online tool.