earthing up my enthusiastic potatoes

I apologise for any repetition... I seem to remember reading answers to a similar question a while back but can't find the posts now.

I'm growing some potatoes (first earlies and some main crops) in compost bags.  The first earlies came up, so I covered the leaves with soil... but they grow at an amazing rate and keep poking through.  I was just adding more soil every day to keep them covered, but seem to remember reading that you're not supposed to earth up too much because the leaves need some light, so I've let them grow a bit.   

So how much is "too much"?  can anyone give me an idea of how much soil to cover them with, and how often?

Keep adding extra compost as the tops grow, just leaving a few leaves showing each time until the bag is full.

No point in completly burying the leaves. Although they will no doubt quickly re-emerge the green leaves are getting energy from the sun in addition to the energy from the seed potato.

I earth mine up in their pots covering the leaves usually ievery other afternoon, but same difference as leaving a few leaves showing I guess cos they spend the next day in the sun where they have popped through.

Ok brilliant,  I'll let them get a bit bigger each time before I earth up then.

Thankyou all! :-)