The imp's description on Roll20 suggests that an imp could turn invisible at will and as often as it has the time for (every turn).
Invisibility: The imp magically turns invisible until it attacks, or until its Concentration ends (as if concentrating on a spell). Any Equipment the imp wears or carries is invisible with it.
I'm asking this because I'm allowing a warlock in my game to have an imp as familiar through the find familiar spell (without Pact of the Chain), but the description doesn't state anything about limitations regarding the frequency of turning invisible. Are there any?