This question came up recently and it's primary focus is to provide market research information to the querent.
I'm unclear if this is similar to problem player/interpersonal crossover, but some of this question seems very much out of our scope.
At a high level, I think the question is currently too broad. The three questions listed are different enough that each should have it's own. But looking at them more carefully, the first one seems outside of our scope as it entirely lives within the world of market data. The only interaction it has with RPGs is the overall topic.
The 2nd and 3rd questions are more on topic, but it's unclear how it can be answered meaningfully to be useful data for the querent(or others finding the question.)
I'm not entirely sure all of the contents of that question are within our domain here.
The first question:
In which countries is it more usual and how common is it in those countries?
This one seems very broad and opinion-based. How are 'usual' and 'common' defined? How does one answer this with objective data that would be useful? This seems more like a general market research business inquiry then something we can, should, can answer well here (unless we have more definitions to narrow the scope of an answer.)
What is the usual length of a gaming session with a hired GM?
This is answerable, but I'm also not sure about how useful it is. Depending on the potential data set, having a few respondents isn't a good indicator for an overall average time spent to determine business viability.
What is the average cost for this type of service?
This can also be answered by users here who have utilized these services. But again, it's a limited dataset that may be not all that useful in business planning purposes.
Or are none of the above the question and OP is just asking where to find this data?
In which case, this is a question about how to do market research. Folks may be able to answer more specifically here given the association with the end-user for this type of service.
Are questions about general market research on topic?