I came from a pure C# background about a year ago and I found I was able to get up to be productive within a month and feel comfortable within about 3. The major sticking points for me have been understanding the iOS and Android development process and their respective APIs. I think if you've previously developed iOS applications and understand the view/viewcontroller and application lifecycle, the learning curve won't be huge. Xamarin does a great job of making you feel like you're developing natively in c#, but, as someone entirely new to iOS development, the native APIs just feel odd.