> How much time would it take for a smart, competent and self motivated person to get up to speed on this new but related field?

This depends on the position; smarts are not a replacement for (domain) experience, because certain knowledge can be learned only through trial and error and/or through long-term exposure to a domain (in other words, experience).

I believe that domain experience is the more necessary the higher level the position is (e.g. seniors who can decide on architecture, leads, architects...).

Having said that, I certainly agree that in the (hiring) industry it's common to be excessively attached to the specific tools used; I also suspect that smaller companies tend to think like this more than bigger ones.

I think it probably depends on what the domain is. If the position is highly technical in nature then I would agree. However, I do think that beyond a certain point the ability to become highly competent on the fly in a new domain is extremely valuable. If the person will be giving input on business strategy, tech pipeline, then the ability to move laterally and adopt new ways of doing things should be highly valued.

The programming language and any platform SDKs (like Android) are the least interesting aspect of this. The history behind any existing applications, libraries, and customized frameworks is far more important to consider. Someone who is engaged in learning Android development systems from scratch is going to be less effective at understanding why decisions were made the way they were, and therefore will definitely be less efficient at leading an effort than someone who already knows the platform well.

> How much time would it take for a smart, competent and self motivated person to get up to speed on this new but related field?

Hiring someone willing to learn a new field filters for generalists and people who need jobs. Neither of those qualities are bad.

Specialists, or people in love with a stack, don't want to use 'some other tech'. As a senior, I pick the stack, because I have an idea where the industry's at and what's best for the type of software I'm experienced with making.

These hidden filters are interesting.

Yes, let’s build the Android app on iOS :)

> How much time would it take for a smart, competent and self motivated person to get up to speed on this new but related field?

I think the more appropriate question is about the relative effectiveness of someone who has been leading Android development teams for 10 years and someone with the same experience but in a different (but related) field.