This time it is from Conservatives Associations and activists at grassroots local level. May was informed on Monday and the vote will take place in 28 days, after the local and EU elections. By that time the whole world and his dog will know how much damage May has done to the Tories. The result will be announced in early June.
The vote is not binding, but if it goes against May she will probably have to resign immediately. That would inevitably lead to another request to delay Brexit and also a GE.