Upvote:3
This UK govt page notes that the fee charged depends on visa duration. The text is:
Fees
A Standard Visitor visa costs £95.
The fee for a long-term Standard Visitor visa depends on its length:
2 years - £361
5 years - £655
10 years - £822
When you get to the "payments" page, you'll have to choose the amount of the payment, and thereby specify the visa duration you seek.