How much time before doing 1st

I say there is no official time....  When you can do it, do it! 

If you're "crazy", according to other "experts", you can do what I did and do it in your first season.

But seriously, one of the biggest considerations you have to make is how much time are you willing to put in.  I don't know how much training you did for your two sprints, but it would be safe to say that you would have to put in at least twice as many hours if you want to step up to an Ironman.

The point is that only you can answer the question of when.  There is no specific time that you are ready.  It's a matter of deciding on the race, commiting to it and training your *** off!!  Goodluck.

Like Rick mentioned, if you have a decent base in either running or cycling, you'll definitely be ahead of the game.  I did a 1/2 IM in my first year and a full IM in my second, but I have a huge cycling base (30 years) behind me.  Century rides to me are like 50 milers to a marjority of cyclists.  A lot depends on your overall fitness level, and how good you are at doing something aerobic for long periods of time.  I've done backpacking trips where I've hiked over several mountains, going from 5 AM until 8 PM.  Slow and steady, but grinding it out, just like an Ironman.  When you can convince your body that it can go the distance, and you follow up with good sound nutrition and hydration over the same time period, this type of cross training is good for your Ironman.