I'd give it at least 3 months and really try to assess whether the discomfort was due to the job being a sucky situation or me being challenging/experiencing natural discomfort after a big change, i.e. getting the job, or even potentially due to some outside factor. I'd also definitely talk things over with friends to get some outside perspective.
After that, if I felt the same way, if I had enough money to survive a year without working I'd leave immediately, take time off, then prep for interviews and go for a better job. However this is very situation dependent, I am a single man with no (serious) responsibilities, ymmv.
After that, if I felt the same way, if I had enough money to survive a year without working I'd leave immediately, take time off, then prep for interviews and go for a better job. However this is very situation dependent, I am a single man with no (serious) responsibilities, ymmv.