How soon after starting new job is it socially acceptable to move on?

I started a new job 4 months ago which was given to me as a mutually-beneficial favour. I'd been working for a national organisation for 2 years and wanted to relocate, and my new manager was looking for an experienced person to fill a vacancy.

The new role involves managing a lot of people, which I knew when I took it on. I've inherited 150+ unhappy staff (they're not unhappy with me specifically, but fed up in general) and it's wearing me down. I'm taking a lot of leave just to get a break from being in the office as opposed to going on vacation, which I've never had to do before.

My intention was to start applying for promotions, which do not necessarily involve management, in the summer as I've been at this level for a few years but now I think I'd like to move on sooner than later. How soon is acceptable to start applying? The way my organisation works (which I have no intention of leaving) is your manager gets an email every time you submit an application. They can't stop me from applying, but I don't want to offend them.

I should add that I get on great with my boss, but I think the expectation from them and their boss is that I'd be there for a couple of years - I may be wrong as neither they or I gave that undertaking, but they expect people to stick around in a role for a lot longer here.

Thoughts on how long I have to leave it? Is January too soon, as I've seen some jobs I'd like to go for.

So you're really looking for a new position within the company rather than to move on.

How does it benefit anyone for you to stay in this current position when it's making you so miserable that you have to take leave after 4 months? Has it occurred to you that this may not be working out for your boss either? It might be a good idea to change positions before your reputation takes a hit.

It happens ALL THE TIME in the working world that people get into a new job and find that it doesn't quite fit them. It's nothing to be ashamed of. A lot of people will force themselves to stay in unhappy situations for fear that they'll be judged for leaving. Your case is different since you want to stay with the company. I would talk to your boss and tell him/her very nicely that the position doesn't seem to be a good fit, the staff seem very unhappy and it's wearing you down, and you're considering applying for other positions within the company. And ask your boss how they would feel about that.

I think it would be disrespectful to simply apply for other positions without talking to your boss, especially since you know they will find out.

Honestly, I think anything before 6 months is insulting. You should try for 1 year.