IMHO: In most cases, dedicating 50% of your time to marketing is a good idea. The notable exceptions would be:
* Your growth model relies on virality. (In which case, as long as you have some traffic, that should be enough to test your model and potentially start growing).
* Your growth model relies on SEO (in which case, you should spend time on that, not direct marketing)
* Your growth model relies on referrals. (Think of a contractor who does no marketing himself, he just incentivizes his clients to refer new clients)
* Your business model doesn't rely on user growth (hard to think of an example here, but I'm sure it exists).
But in most cases, you've got to market the product aggressively. Think of how many millions of sites exist (and are vying for your users' attention). Without some effort put into marketing, there's little reason to assume a user will visit your site at all.
That being said, you should do what you're good at. If you feel like the product needs to be marketed heavily (and you don't enjoy that kind of work), then maybe it's worth finding a co-founder who enjoys that part.