As Mike says, you need to cut out the rusty metal and weld in replacement. The sill is a structural part, and so needs to be strong, and any rust repairs for the MOT have to be fully welded.
How long it will take (and so how much it will cost) will depend on several things:
Can you get pre-made replacement panels, or will they have to be fabricated?
You need to cut away the rusted material.
This will need to be replaced and it must be welded into place - pop rivets and screws are not sufficient. The repair should provide the same structural strength as the original.
Replacement sills either complete or sections are available. If not a good shop will make suitable sections as needed.
Any asking of ...