How long before I can move on to a different phase?
How long you wait doesn't matter[1].


  I decided after a minute or so to play as creature so i had one to block. I then said shortly after probably 3-5 seconds i will attack with my inspired creature. 


Let's assume you retained priority after casting your creature[2].

At this point, you're still in your main phase -- the stack isn't even empty -- but you can still say "I attack with my inspired creature" as you did. 

What you are doing is proposing a shortcut consist of: Both remaining players passing priority until the Declare Attackers Step, then declaring your inspired creature as an attacker.

He can accept your shortcut in whole, or he can interrupt it at any point. In this case, he let you pass priority, then he cast a Counter spell. What he did is perfectly legitimate.


  Now as i understand when i move on to attacking phase he can't then play an instant to counter me casting my creature spell as that phase has passed (correct me if i am wrong). 


Once the creature spell resolved, it's too late to counter it. But nothing in what you said indicates this has happened. You can ask "Resolve?" to make this happen[3].




Exception: Playing unduly slow can result in penalties.
By the rules, you regain priority after casting a spell if you cast it when you had priority. However, in tournaments, this rule is modified. You are assumed to have passed priority after casting a spell unless you have explicitly retained it.

If you hadn't retained priority, you wouldn't be able to do anything, including proposing a shortcut. Your statement that you were attacking would have been completely meaningless.
If you are in a tournament and you didn't explicitly retain priority, he has priority and he must do something. Usually, saying "pass" or "ok". This moves the game forward, allowing the spell to resolve. Asking "resolve?" would remind him of this.

If you are outside of a tournament or if you explicitly retained priority, saying "Resolve?" proposes the following shortcut: All players pass priority until the spell resolves.