It used to be described quite clearly in the old Standards Document 20, but that's been withdrawn.....
Standards Document 19v4 para 3.6.6 is about all the real guidance there is these days.
I use a first leg of 40-50 nm and act as an 'intelligent passenger'. I will point out any other aircraft I see and just ask the applicant to tell me the ETA for the turning point and describe what he/she's doing if correcting a track error or updating the ETA.
The second leg is planned as 50-60nm; however, after 10 min the 'weather ahead deteriorates' and the applicant is then asked to divert to somewhere I indicate to him/her - but only when he/she is ready.