You can remain in Belize for up to 6 months if you report to Immigrations every 30 days to renew your 30-day tourist visa. After 6 months, you must leave Belize, if only for a short time. Go back home for a quick visit; or maybe travel to Panama for a week. When you get your 30-day renewal, ask the agent there how long you must be gone after 6 months. I think it is only a few days - but do ask first.
I always get my passport 30-day renewal done at the Immigrations Office in Dangriga. They are so NICE there! Everybody is nice in Dangriga! .
Three points:
1) I am under the impression that after 6-thirty day extensions the price for the extension just doubled from BZD$50 to BZD$100, without needing to leave the country.
We have never stayed more than 4 months at a time because of needing to do things back in Texas.
2) It is possible to request and get a 60 day or 90 day extension just by asking and paying the total fee due for the time requested. HOWEVER each office seems to have their own set of rules on this. At the Big Creek port office (Independence) Only the head officer is allowed to sign the extension beyond 30 days. The last time over we requested and received a 60 day extension. (We did not need the 90 day). A week later I mentioned this to an acquaintance going after her 5th extension and she received the 90 day one. She thanked me the next time we saw her. They do ask why you need it, and we simply said we were old and is was hard on us making the trip from Placencia.
3) I know two people that reside in Placencia, went to Dangriga last month to get their 30 day extensions, get chewed out for bothering them in that office instead of going to Big Creek where according to the officer they were "supposed" to go.
They went to Dangriga because they had other things to do there and just timed it to do everything in one trip. They did get their extension, but were told not to do it again....
So like many things in Belize, your millage may vary. 
Good news, if true for folks in Placencia. From two local sources, I was told Immigration was going to open an office in Placencia, so we wont need to make the Hokey Pokey run to Mango Creek, then a cab ride to Big Creek. (If it happens)Â 