How long on average would it take to play Runebound solo?

If you play at a brisk pace and don't linger around cities waiting to go shopping, you should be able to get a game in in under an hour easily.

When I play solo, I use my own rule that being knocked out ends the game, so game time can be as short as five minutes. This gives me plenty of time to start another game right up. Some use the doom track for solo play; I don't. I just use the travel hazards optional rule and my knockout rule.

If you like to build the story for your character as you play, I reckon you could stretch out a solo game to a couple hours, if you were content to work out a storyline as you play.

Ronan's story line usually ends with him getting a slap across the face from a female hero at a local tavern...

I play with the doom track, otherwise it won't be challenging enough. You can't get knocked out or you won't make it. You also need to escape Challenges that make the Track advance and don't give enough rewards.

Each solo games lasts between 30 and 45 minutes, depending on the cards you draw. You need to go back to cities quite often to gradually spend your money and improve your ability. There's not much challenge as you have every market just for you, so I don't see very much purpose if you don't play with a restricted timespan, as given by the Doom Track. Except of course if you like Runebound as a sort of role-playing game as Torbal does.

Playing solo is interesting to try out the Heroes and understand their strengths and weaknesses. Same for Items and Challenge cards. If you play some games solo and your friends don't you will be in significant advantage during the next games. I've won every game since I got familiar with card content and advancement dynamics, despite being knocked out by some nasty challenges/bad luck combinations. I also managed to win with difficult Characters like Ispher and Runewitch Astarra. The only one I can't manage at all is Sir Valadir. He's really weak compared to others!

When I play solo I generally play four characters, so that stretches the game out to about 2.5 hours. However, I only set up the game when I have plenty of time to play so I don't have to rush through it.

Hey leave poor Valadir alone! He is an artefact from 1st edition that can still put up a fight with a variant Market Rule that favors his ability to hire for cheap.