I made an imperial stout last year with 8oz of molasses (and 8oz of date sugar). It was a high OG at 1.084 and finished at 8.3% ABV. Even with all that alcohol, dark malt and hops, the molasses was very present - even after aging for 6 months. It wasn't overwhelming, but when people tasted it one of the tasting notes I consistently got was "molasses flavor". If you like that then 8-10 oz is fine but if you want a subtler character maybe 4oz? Only did this once and like I said I also added 8oz of date sugar which does have a molasses flavor also, so that could have put it over.