In Silicon Valley the typical investment vehicle has become the SAFE note - even for early stage VC's. The Alchemist SAFE is a good template that's founder friendly.
It depends on the terms of the note itself. If it is forced to convert on defined cap, then maturity date isn't as important for founders in most cases. If it converts on current "fair market price", then you would want to have it at least year and a half out, preferably even a bit further.
You can also talk to a lawyer first. It is more founder friendly if no forced conversion at maturity. Because if your company is doing really well, and you never need to raise a priced round (to trigger qualified financing), you never have to convert. And investors won’t call the note, because they obviously aren’t interested in the payment back with interest (which is meaningless returns, they want the upside of the shares). And if your company isn’t doing well, calling the note is meaningless also, while it can shut your company down, then they don’t get anything anyways. Usually they’ll have to wait.