A personal-loan due date does not have to be a rule for the borrower.
In ordinary family and friend lending, the date is often more useful as an internal reminder for the lender. It gives an indefinite loan a moment when it becomes visible again.
The problem is disappearance from attention
Most informal loans do not fail because somebody misses an agreed deadline. They fail because there was no next moment to think about the object, and eventually no clear memory of where it went.
A date solves a smaller problem: it tells your system when to put the object back in front of you.
Pick a date that matches your own need
If you need the ladder for a project next Saturday, set the date before then. If there is no urgency, choose a loose date a few weeks or months out simply to review the loan.
When that date arrives, the action may be nothing more than noticing that the book is still elsewhere. You can extend the date, ask for it back, or leave the loan alone.
Do not manufacture false urgency
Due dates on every casual loan can become noise. Use one when it changes something for you: when you will need the object, when enough time has passed that forgetting becomes likely, or when you want the outstanding loan returned to attention.
This is different from a promise
If the borrower actually agrees to return something by Friday, record Friday. That is different from choosing Friday because you want your own phone to remind you that the object remains elsewhere.
The system can be precise without making the relationship formal.
A relevant Ulix tool
Loan It
Loan It records borrowed and due dates for outstanding loans. A due date can therefore function as the lender's own resurfacing date, whether or not it represents a deadline agreed with the borrower.