If you are relying on Silicon Studios alone to add features like ads, IAPs, etc. then it will be a long time (if ever).
No, I don't think Silicon Studios has to implement all infrastructure. It's fine if it's implemented by some well-known third parties in a way that is integratable with Xenko. I have no experience in Xamarin on mobile, so I don't know the gap between “implemented as .NET library” and “known to actually work in production”.
@Kryptos says: “If there is a .Net API then yes.” But if something is implemented as .NET library this does not necessarily mean that this works on mobile. For example, some managed library may reveal to be working only on desktop Windows, because it just
PInvokes WinAPI calls. Better to know such pitfalls as early as possible.
Unity only added that stuff in the last year or two
I don't use Unity's API for networking, purchases, or ads; preferring third-party vendors. (Unity's APIs are in general poorly designed. So are third-party's API for “gamemakers”, though.)
What I want to hear is not only “I've heard Photon Cloud provides API for .NET, not only for Unity”. But rather: “I've actually succeeded using Photon Cloud on Xamarin.Android in my game made with Xenko”.