My main character is lvl 52 with over 250 hours in. After I completed the main quest as well as quest that I felt were relevant to the character I chose to start another. My second character is skill wise is completely different so I could experience different things and have a different perspective on main quest. I'm not sure which character I'll use for Dlc yet though.

I don't have a rule for waiting, I just make them as I feel like it... that's why I have a 51, a 20, an 11, then five that are 1-8. I just like running one main chracter (the 51), with a seconday (the 20), then ones that I just did for some fun (all the rest).

I start them when I feel like it. If the current ones fails or get's trapped in dungeons and can't get out. For fun. If things go wrong. Because I want to try things out. Lot's of reasons.
If I feel like making a new one I do and I sometimes run 2 of more at the same time.
I once made a female orc warrior to play properly because I was doing the hideous TG trophies with a unfortunate female kadjitt, the whole save was quaratined and only approachable by wearing a hazmat suit. All she did was get the TG trophies over and done with as fast as possible. I deleted her the moment I got them.
The orc was the proper played character.

I sometimes make characters as insurance, in case my trophy run one goes wrong.

Level 52, 500+ hours, still playing with her. I guess I'm having too much fun with her. Finished the Mages' College (for restoration magic) and the Companions, just "had a talk" with Mercer for the Thieves' Guild, done the first quest for the Bards' College (won't do more there with this character), and now I still have to meet Odah for the MQ. Still planning on a pilgrimage of Kyne's finest sights, and start to mark my territory as a werewolf. Oh, and buying that house at Solitude for Barenziah's stone. It's difficult to rake together 25,000 gold if you've limited yourself to only get money from creating silver jewellery.

But I've got two other characters in mind already, 'though they need some fleshing out. But I guess that'll come with playing as it did with this first one. So maybe I should start my thalmor spy at last.

Level 50 250hrs and just now thinking about a new one. I will I just have not decided how I want to do it just yet.

I played Oblivion with just one character and pretty much did everything with him (a Dark elf fighter/mage). For Skyrim I decided I wanted to explore some more playstyles, so I've created three characters in succession, representing the three archetypal characters (mage/warrior/sneak).

I tend to want to start afresh when my current character starts creaming everything, even on master difficulty, which tends to occur close to level 40. When the challenge goes away, it's less fun, so even if major quest lines are unfinished, I tend to stop and start the new character. That always reinvigorates my interest, and I love the fact that the game allows you to go in completely different directions with each character, without repeating any missions save the initial one in Helgen (unless you really must have shouts).

I did 150 hours with my mage, 50 hours with my warrior, and I'm probably at 40 hours or so with my sneak (who is proving to be my favorite, to my surprise).