I believe payment is required for a period of 5 years for both applicants and dependents. It’s worth noting that Immigration Health Surcharge rates may potentially rise in the future (Almost double) , making an earlier application advisable. In my case, all charges were invoiced in GBP.
You don’t have to pay for the full 5 years in advance, you can pay for less than that (e.g 3yrs ) and renew after its expiration, same applies to the dependants