How long do we have to wait before we can foreclose on a land contract?

You should be able to foreclose, however the final answer depends on what the land contract says. Without reading the contract no attorney will be able to give you a better answer than that.

Foreclosure or more likely forfeiture of a land contract is a tricky subject and can get very involved.check out http://courts.mi.gov/Self-help/center/casetype/Pages/LandContractSH.aspx just because the tenant is late does not give you an automatic right to foreclose depending upon the terms of the land contract.