In order to not be considered a gift, the appropriate Applicable Federal Rate must be charged. This varies by the term of the loan; short term is 0-3 years, medium is >3-9, long is >9+.
Investopedia discusses this a bit more, particularly including the time periods.
In your case, of course check the AFR for the month in which it's lent, but it will be around 0.45% annually compounded (give or take a bit). They've been going up lately, so pay attention (or charge a bit more to be safe).