On 401K loans the maximum you can borrow at any one time is 50K or half the vested balance but never more than 50K.
So if the value of your 401K is 200K, the most you could borrow at any one time is 50K. With my 401K they offer up to 3 loans at a time max, so once you have paid a loan down you can get a new one for whatever the balance owed at that time is subtracted from 50K. So if you had 2 outstanding loans for say 25K you could take out another loan for up to 25K. The loan repayments are deducted directly form your paycheck.
The loans can be repayed for up to a 5 year term unless the funds are explicitly for a mortgage down payment, which can then be paid back for up to a 30 year term I believe, ofcourse all plans are a little different.
If you leave your job or are terminated the loan balances become due immediatly. Im not sure what happens in that case if you dont have funds to pay back full balance.
