I think there might be two issues with the turf - one, how long it takes to "settle", and two, whether it has been treated with chemicals.
Personally I wouldn't put the guinea pigs out in a run on the grass for a good while yet, as it is still cold and damp, and I wouldn't do it until I myself felt warm and comfortable outside - especially if they normally live indoors. Guinea pigs are happy in the same sorts of temperatures as we are, and don't cope well with sudden changes in temperature.
Another thing to be aware of is that the ground is often very damp, and guinea pigs are very low-slung, without much "clearance" from the ground! It wouldn't be very nice for them to be on damp grass, not to mention possibly injurious to their health!
They should always have access to hay, even if they are being fed grass. Good quality hay is absolutely vital to keep their digestions and teeth in good order. It should make up the bulk of their diet.
When you do put them out on the grass, remember that too much all at once could easily upset their tummies, especially the rich new Spring grass, so go very carefully at first, and strictly limit their time out until they are used to it. Or you could just cut a very small amount and start giving it to them in their cage or hutch, to get them used to it before you put them out.
Depending on what the weather does, and your local conditions, I would think maybe May or June might be a good time to start putting the piggies in a run outside, but I'd err on the side of caution myself! That should also give plenty of time for any chemicals in the turf to have worn off, I would think. Hope this is some help.
