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?
Best Answer
There is no limit to the frequency of use.
The imp's Invisibility feature is listed under Actions, and there is no indication that it has a limited number of uses. This means it can be used as often as the imp gets to take an action. The only restrictions are that it requires concentration to maintain the effect and the effect ends immediately after making an attack.
A normal familiar obtained through the find familiar spell can't attack anyway, so in your Warlock's scenario it's very difficult for the imp to really abuse this.
In the case of a familiar obtained by a Warlock through the Pact of the Chain feature, however, it would be able to attack. In that case, the "cost" that would limit abuse is that the Warlock would have to give up one of their own attacks to let the imp make one. This wouldn't actually limit the number of uses, but it would impose a restriction that limits its practical effectiveness.