I don't have a "definitive" answer if there is one. I don't think it's in the spirit of Holacracy to let a LL say:
"Hey you! Spend 20% time on role A and 80% on role B".
Most importantly because that would be predicting and controlling and why would you want to do that?
However, it's encouraged (constitution quotation needed ) that LL's discuss whenever roles have trouble defining priorities because of resource issues and the roles can't fix it themselves. This process has the same outcome as "do 20% this, 80% that", but has a different feel / route to it, IMHO.
In practice, we have had LL's say things like "don't spend more than X % of your time on Role A". IMO this is OK to do especially if the LL thinks that this in effect is about role fit. Eg. "if you spend more than 1 day a week doing this role, you're doing it wrong".