It's not a silly question There is no such thing. I read somewhere recently that 90% of young couples fall pregnant within the first year of trying (provided fertility etc is normal, which at this stage you can assume it is) - I think it's best to relax as much as possible, enjoy the sex (don't feel pressured every time you DTD to think 'could this be the time?') and let your body get into the groove. If you're really concerned, talk to your GP, but I've had plenty of friends in their 20's told by their docs to give themselves a year of trying before they start getting too concerned.

Good luck with it - I hope it happens for you soon x