As a previous playtester for Malhavoc, I can answer this somewhat.
The designer states what he wants playtested. It can be a particular rule, a class at the low end to all the way to a entire 'system' (like Arcana Unearthed/Arcana Evolved).
The designer isn't going to dictate when, how and for how long your group meets. What he wants is a good, well tested answers to the parameters set in the playtest. Well tested as in more than just kicking the tires by trying whatever mechanic or rule in a variety of conditions.
As playtesters, the amount of time and effort you put in is up to you. Obviously, the more time and effort, the better as far as the designer is concerned.
Playtests are done to deadlines, the reality of playtesting. Obviously, a group which meets twice a week to playtest and torture test mechanics/rules within a given time frame will have a better feel for answering the designers playtest concerns that a group which meets only once every two weeks.
So, no - the designer is not going to dictate terms on how often your group meets and how long each session is. The designer may ask as part of the application process on average how often your group meets and for how long to see which groups are suitable for the playtest requirements.
Of course, I haven't playtested for WOTC previously and may be totally off base on what I stated above as companies may differ in their playtest methodologies.
