Fees for Darksend time seem less useful than fees for Darksend amounts. Depending on how many people are anonymising their DRK, 24hrs may or may not be enough, but if you could pay a fixed fee upfront to anonymise a fixed amount of DRK, that would be better IMO.
Solution 1: Fixed Fee Session Model
You enter how many DRK you wish to anonymize and it calculates the fee and will give you access to the masternode network until your set amount of DRK has been anonymized to your requested number of rounds. After that you are locked out and need to pay the fee again to access the masternodes based on how many DRK you want to mix.
This way its a guaranteed delivery of service. However I think it has a major drawback...There might be a way that the fixed fee could be tracked to your mixing input (as its fixed and needs to be tied with your input signature) and compromise your anonymity.
Solution 2: Subscription Model
As I see it the subscription model will just give you a "token" to say you can mix whatever you want for X period of time and not tie your inputs to that subscription hence upholding anonymity. Drawback here is if there isnt enough liquidity during the time period you have been granted, you risk not matching inputs and spending 0.1DRK for nothing. Although talking with Evan the liquidity will solve itself over time as more users mix.
Not sure if ive been smoking too much but let me know if im way off
but in an amusing way.