I imagine geting prestige 10 will take around 4k hours total, assuming you're good at it.
I also thought about making my entire collection shiny, but first I need to complete it. I was quite sure I will be able to get all cards after this massive scrap fest, but still Gwent has so many cards I am missing over 20 legendaries.
Nah, not at all.
I got to Prestige 10 Lv60 a few months ago, when i had around 3k hours BUT the 1st 1k hours were on beta, and the progress there was slower, capped and hindered by resets, so when HC started, players that were Lv100 (max) on beta, became prestige 1, level 40.
This might seem like a good headstart, but it wasnt equivalent of the time committed on betas.
So, i would say 1500-2500 hours is a more accurate estimate for a player that only played in Gwent HC, starting from scratch all the way to max (which is supposed to give you a title but the contract has been bugged for months, maybe years
)
Regarding the premium collection, i actually finished mine before i even reached prestige 10, and never even spent a single cent on kegs or powder, however i had a huge amount of resources from betas and also did an optimized behaviour to speed up the process - give priority to transmuting legendaries, never milling cards i didnt have dupes, etc.