I vaccuum mine once every couple weeks - every 2 weeks at the most, once a month at the least. If it's heavily planted it can be harder to vaccuum as you have to work around uprooting plants. I'm always amazed at how much crap comes up when I vaccuum, so I don't recommend skipping it altogether. You can vaccuum up as much or as little as you want. If you're concerning about taking out too much water, vaccum half the gravel one day, and 2 weeks later vaccuum the other half.
Also, how many cories do you have in there? I hope at least 6 with that tank size. I only ask because sometimes betta people don't know much about other fish, and cories are shoaling fish. Just a thought, sorry if that is information you already know!