The thing about open mics / jam sessions is that the audience will comprise almost entirely musos waiting to play. In most cases that means that they'll be nursing a soda & lime, so even if the place is full there'll not be much to show for it from a landlord's POV. For an open mic to be viable, you'll probably need to attract local musos who might in due course become regulars at the pub on non-music nights.
Whether you go open mic or bands, the key will anyway be the promotion. Posters, websites, gig listing, social media, you name it. And that never stops. Ever.