Budgie,
When: Whenever you want it, when you are VFR usually OCTA.
How: In radar coverage, the service will be terminated if you drop off radar coverage, but a controller will tend to keep an eye out for your track in case you re-appear.
Can't speak for other controllers but I do sectors that include class g directed traffic information. I'd prefer that vfr call me and ask for flight following. Makes it easier to provide info to both you and the high performance RPT that come through.
Unfortunately a high profile person that 'dabbles' in aviation has told vfr's to shut up and say nothing on centre frequency which leads, in my opinion to them not listening as well.
Don't think I've ever knocked back a request for Flight Following, in fact I've asked vfr's exiting class c airspace if they want to continue on a Flight Following basis. I'd say most controllers would be the same?
It does make it a hell of a lot easier if you have submitted a plan though
