For me, when I'm looking at applications, it depends on a few things.
1) Is this the nth time you've changed jobs in the last few years? (normally signals that you'll leave pretty sharpish, either because you don't work well or you're unbearable to work with)
2) Does the sector that you're coming from relate to the one you're applying too? (For instance, going from a care home to the NHS is not massively surprising after a few months. Going to the NHS from a vets surgery is slightly odd and could indicate a lack of interest)
3) Are you moving location or similar reasonable need to move job so quickly?
4) What reason have you put for leaving your last job? (If you've put 'Manager was awful and we didn't get along', that's an alarm bell. If you've put that you want to progress and feel you can't do it there as there's no training facilities, then that's acceptable).
I would say at least 6 months in a job, but if you really hate it and it's making you ill then go as soon as you can and justify it in an appropriate manner. Don't slag your workplace off, but don't say that you're leaving because you can't develop if you can as when your reference comes through they will be honest about why they think you're leaving!