Lending short term money, help with rates

[James Gentry]

My wife and I are looking to lend flipper friends money so they can close on two wholesale homes.  The term is 1 to 2 weeks.  What interest rate should we charge for this short term loan?  If they state that it will take two weeks and it extends to four, should we have stipulations that address the extended term period?  Thank you for your input.