We have only been clearing as a 25 man guild for 6 weeks, Yet I am IL 502 right now. I'm thinking that the learning curve in normal won't be that high and it will be relativly easy to do on normal in the first week tbh.
Clearing new content vs old content is completely different, we killed will of the emperor with an average ilvl less then 470, i'd assume since you guys just started progression you were probably 480 +, you also had experience in lfr at least + kill videos and strategies to watch, first week raiders have non of that.
Top guilds i assume will clear normal in first week (not sure if Ra-den is available on normal). Heroic is a 12 boss progression, i'd expect 2 weeks depending on difficulty (not including ra-den). Ra-den i think will fall a week after lei shen for top 5 guilds, Reasoning being is 30 wipes for a HC boss with no information and no strategy may be too difficult, the reason i say 2 weeks for the other 12 is that assuming the bosses are difficult, a week may be too short of a time to overcome all bosses.