This library, taking advantage of Apple's official rating request system, gives you some control over when you ask the system to request a review. Since you only get so many reviews, it's become even more important to ask at the proper time.
GetRated will ensure that a number of configurable requirements are met before prompting. These include daysUntilFirstPrompt, daysUntilFuturePrompts, minimumDaysUntilPromptAfterVersionUpdate, usesUntilPrompt and eventsUntilPrompt.
GetRated from Neil Morton