I can't seem to remember where I read this from (so don't quote me) but I think either Lorre or Prady said that they will explore further into the Leonard/Penny relationship in season 6 but Penny is the one who has to grow up emotionally before their relationship reaches a whole new level. As to how long it will take for Penny to grow up is not known. I mean, I personally don't like girls like Penny- those to take their relationship for granted and think that their bf can't do any better or won't break up with them. So it may take a while for Penny to realise that she is the one who is stalling the relationship not Leonard because he knows what he wants.
Judging by what has happened so far and reading the spoliers, I think they are headed for a break-up (hopefully they stay apart for a few seasons) and the introduction of Alex who is more Leonard's intellectual equal and who shows interest in him first and obviously admires him, is perfect fit.
I just don't want the writers to rush this storyline but rather have Leonard move on and have Penny have a realisation that she is not what she thinks she is and she can't do better than Leonard.