As InFerrumVeritas said, slowing time down for yourself would just mean that you are faster relative to the rest of the world, although unlike regular speed powers, it will affect everything you do: you'll move faster, react faster, heal faster, learn faster (you'll also starve faster, so remember to put the next dinner in the oven as soon as you've finished the last one, because it'll take much longer to cook from your perspective).
I'd model it by calculating shifts for all these effects, whichever one it is that you actually want to do, and requiring the ritual to meet that.
Speeding time up is mostly going to be a way of getting around really boring waits (or not suffocating if you're stuck in a safe for 48 hours before someone comes and gets you out). In terms of the reasons that costs and shifts are usually measured, the good you'll get out of this manipulation doesn't amount to much; I'd measure it equivalent to a ritual allowing you to go without food and air for longer. Again, amass the shifts for both, even if one is only an issue.
Wouldn't it be funny if travelling back in time was actually impossible, and the White Council only banned it because it meant that warlocks would think it was, and a certain proportion of them would be stuck researching this dead end instead of doing any damage?
