Gym without diet? (how effective, is it worth it?)

Hi. I really appreciate all of your answers.

I'm not serious on bodybuilding, living the gym life, jerking off to my biceps etc etc. I started going to the gym to fill some time in and fix my back, but while I'm at it, of course I'd like to see some results.

I don't want to get mad ripped or something like that, but a nice, defined body with muscle would be desired. Since I'm not all into it, I don't care much for changing my diet and I certainly don't want to give up drinking alcohol.

So be honest, what kind of results could I expect and in how much time without giving extra consideration to my diet? Btw, I'm not fat or skinny, I'm about the right weight with a bit of a belly, but not that much, if that helps. I am very thankful for any helpful replies.