How long do i pay for 50 gb plan upgrade using iCloud

You pay monthly for as long as you want to use the storage space. It is similar to renting physical storage space or renting an apartment in that you pay each month as long as you want to use the space. Cancel by downgrading to the free 5GB plan. See instructions in: Downgrade your iCloud storage - Apple Support

The following may help if you want to see what is using the space and to possibly reduce it: Manage your iCloud storage - Apple Support