Timeline to build a fully custom theme from Luma?

I’m working for a b2b company and we’re trying to build a frontend for a magento website. I’m wondering how long it’ll take for me to build a fully custom theme extended from luma compared to if we get Hyva Themes? I’m extremely new to Magento and I’m struggling with how just one simple thing can be so complicated to move around from the product page alone. I’m wondering if this is the right way to go?

background from me – worked as a wordpress developer for 6+ years.