It's hard to give a straight answer but I will try!
The short answer
About six to eight weeks assuming planning has been granted and party wall agreed.
The building work itself can be around 6 weeks. However, depending on the specifications especially with bi-fold doors and glass roof this can have a longer lead time depending on the supplier.
The long answer
Assuming that the builders turn up and work solidly throughout and there are no big problems along the way then 6-8 weeks should be enough.
However I have seen/heard of builds take much longer usually because builders weren't reliable or specifications changed.
For example, one build I know of had to redesign the steels in their kitchen half way through because the planners decided that more steel was needed. The client had done everything correctly but simply fell foul of a rule change with the council.
Assuming that you have everything squared away then finding reliable builders is your best bet for a quick finish.
I'd ask to see several (three ideally) of their completed builds and ask the owners directly how long it took from "arrival to dinner party".
What you really want to see is a pattern of rapid solid work with no endless chasing for snagging etc.
Hope all that helps and good luck, you'll love it when it's finished but the process can be a little stressful!