Questions About sunbeds

How many minutes do I need to use a sunbed for to maintain my tan?

I've just got back from holiday and I am fairly tanned obviously, but now I want to maintain my tan, but at the moment the weather in england isnt very good. So I'm going to use a sunbed to maintain my tan. Obviously I don't want to over do it, but how many sessions a week should I use a sunbed and for how long per session?? Please answer the question properly, thanks x