The day on which the dividend is announced, the share becomes
"Cum-Dividend" or CD. The day on which it goes ex-dividend it becomes
"Ex-Dividend" or XD. It remains XD until the date on which the
dividend is paid. The length of time can vary considerably, BP, for
example, went XD on 11th May and paid the dividend on 6th June.
Whitbread, on the other hand, went XD on 4th May and the pay date was
5th July. RSA went XD on 10th March and pay date was 2nd June.
As you can see, there is no set time limit for being XD.