I put my guinea pigs out on the grass at every opportunity. This weather has been awful though, so they've had considerably less run time than what they are used to, but they are out just now as the grass dried and it is not raining - for now.
So long as it is dry, not too cold/windy and not a storm (with one extreme exception), the pigs are out. I cover their runs and they stay out in the rain sometimes (they're completely covered though!). Obviously, if it chucks it down, they go back in at the next opportunity because they run out of dry grass to eat and I can't move them to the next patch as it is soaked.
I always put them out when it is really hot because their hutches get direct sunlight at different points of the day and I can't move them on my own, so they're better off in the runs which are shaded, spacious and airy.
I think the latest they have been out has been a particularly nice October. When they start going out at the start of the year really just depends on the weather. I think they had some run time in February last year!