Obviously the founders has a reason to borrow money from you (second time) but you too were already 75k down and would have wanted it all to succeed. So my gues

Obviously the founders has a reason to borrow money from you (second time) but you too were already 75k down and would have wanted it all to succeed. So my guess is that your 50k further investment was partially in your favor too. So I think if you get back 2-3 times, that should be good enough and you should give them a fixed time frame to pay you back (nothing less than 6 months and nothing more than 1.5 years).