There are tons of questions with people asking how often they can visit Country X:
Can I leave South Africa before my 90 days expire, and return after a few days to stay for another 90 days?
UK Visitor Visa: Required Time or Interval Between Visits?
How soon can I re-enter the USA having stayed for 90 days under the Visa Waiver Program?
Time between visits without a visa
I'm thinking we could get a repeat-visits tag so that it's easy to find one of those questions for each country. What do you think?