When to dig up surprise spuds?

I have spuds growing from peelings in my bins. You may get some spuds, but it depends on the size of the composted spud. If it was a whole spud then you may get some, but if it was a small piece, you will get the foliage but you may only get tiny spuds or none at all.
Have a careful dig around and see.

Such spuds are known also as 'volunteers' (Though I think 'surprise spuds' sounds much better!)
Even though they might give you a few extra potatoes, it's best to destroy them as soon as you see them as one of the main ways potato blight is spread is on volunteers from potato tubers or even skins which were infected in the previous year. I've made this mistake in previous years and suddenly found both new potatoes and then tomatoes affected by blight which has developed on these self-sprouting spuds.

Thanks for replies...will dig them up & chuck the green away...I'll let you know how many pounds of taters I get!! :0/

The size of the original planting has nothing to do with the size of the crop. It is the quality of the soil and the amount of water which determines that. If you want a challenge, when you get a seed potato rather than planting it as it is, wait until every eye begins to grow, then using one of those peelers with a thing for removing eyes, take out a plug of potato including the growing eye. Plant in a small pot of compost with the base of the new shoot just below soil level. Grow on until the pot is full of roots, then plant out in the normal way, earth up and see what happens. This is how these people who claim to get hundreds of pounds of spuds from one tuber, do it.

But back to the question, the problem of blight is only an issue from Volunteers, if there was blight on them in the first place, if clean then no trouble.

One thing about blight, it starts on the leaves and travels to the tubers, remove the leaves and while you get smaller crops, it is at least clear of blight.

Its best to bin em! ive found that peelings and small bits dont come to much at all, normal bought seed spuds grow really well for me