Perhaps a decent rule of thumb is that when you struggle to see the ground over the nose of the aircraft in level flight it's getting "marginal". Frankly, I find that trying to do navigation by chart and features is pretty difficult when you can't see in front. At 2000' in yer average spam can, you can't see nearer than a couple of miles in front so when the ground starts to disappear from the forward view you're still legal, but whether it's sensible is quite another matter.
Let's not deceive ourselves here. The legal limits for VFR are - in the UK - IMHO really low. When the visibility drops towards the legal minimum, the last thing you need to be worrying about is the legality of the flight. If you're only trained for VMC you need to think fast to achieve a safe outcome.