I've done Snowdon in the past but can't for the life of me remember how long it took to get up and down it.
We'd be in 2 groups- both going up Llanberis, then splitting and half doing Rangers and half doing Llanberis.
We'll be doing it in 2 weeks time, so it'll be dark at about 4pm and we want to be down by 3.30 ideally. Is 6 hours a reasonable guess? We did Helvellyn in about 4.