Both, I suppose. But “when” sounds better in my opinion.

In this context, 'when' is used as a conjunctionand means the same as 'if'. They can be used interchangeably. http://learnersdictionary.com/definition/when a — used to say what happens, is true, or can be done in a particular situation When you have no family, you are really on your own. We take the bus to school when it rains. A contestant is disqualified when she disobeys the rules. When and if he comes, you can ask him. = If and when he comes, you can ask him. b — used to ask why or how something is done in the situation that exists Why buy a newspaper when you can read the news online for free? How can he buy the house when he has no money? [=he has no money, so how can he buy the house?]