Depends on various factors, but usually between 10 and 12 months.
I did notice Tron Legacy is on unusually soon even for Sky. It's being advertised now, but was only out in late December/Early Jan..? Unless I am getting mixed up with box office
I would say 6-12 months depending on the film studio. To take this week on Premiere as an example, as pointed out Tron is on which was in cinemas in December, plus Inception is on which was in cinemas last July (but had a long cinema run).
No, you are correct, it was released on Sky Movies Premiere last week. I thought it depended on the agreement between Sky and the film studio - so it varies between Disney, 20th Century Fox, etc.
It depends on factors like school holidays and overall viewing trends too. Releases seem to appear faster generally around the key end-Summer/Autumn period.
They usually have a major family movie or two to plug around school holidays too.