The time taken does depend on the hardness of the rock. Most of the stuff up my way is on sandstone, so it doesn't take that long. Mebbe about 10 mins for an average 3inch cup. NCl Uni's Clive Waddington has timed his efforts and comes up with about the same. If you use metal pick, no more than 2mins.

The amount of time taken to make a nice neat job is obviously greater. There's a bloke did a repro of the St John Lee Stone, (The repro's somewhere in Hexham I think). It's got multiple rings, and he included weathering, so it took him days. You can get much quicker weathering courtesy of mssrs Black and Decker.

I've had a go at igneous stuff, and even with metal pick, I think about half an hour for a decent sized cup would be about right. If those panels at Derrynabhala are on granite, it represents as much affort as the carvings on the Langdale boulders. Of course the whole panel could have been built up over years, if not centuries.