Of course it's impossible to predict, but one can make an educated guess.

Murray is 25 yrs. old, let's assume he can play at this level for another 5 years..

The top contenders for the slams are Murray, Nadal, Djokovic, and Fed, but then there are the "dark horses" such as DelPo, Berdych, etc., who may not be top contenders for a title, but have the ability to derail the title ambitions of the top 4.

Will Murray ever go on a tear like Fed and Djokovic, and win 2 or maybe 3 majors in one season ?

Possible, but very unlikely...

So, best case scenario IMHO is one major every year for the next 5 years, maybe one year with 2, so 6-7 majors total.

Worst case scenario, he becomes a one slam wonder such as the other Andy.