Neat, dumb question - are the tokens you prepay for good forever, or do they expire? And do they provide any assurances or SLA's about speed? (i.e. that in a

Neat, dumb question - are the tokens you prepay for good forever, or do they expire? And do they provide any assurances or SLA's about speed? (i.e. that in a year they won't decide to dole out response tokens to you at a snail's pace)