How long should I wait to ask for a salary raise?
There are many factors that enter into what makes up an "appropriate" period of time.
You are a new graduate - only on the job for a few months. You accepted a salary very recently and now feel you should ask for more. It's unfortunate that you didn't learn how the whole "get a raise" process works at your company before you accepted the offer, but that's in the past.
Presumably you are in your first professional job. Now, look around and learn how things are typically done at your company.
Does your company have a "probationary" period? Often companies put newbies in a special, temporary status that makes it easier for either party to end the relationship at the end of the period. During that probationary period, workers are often paid somewhat less than the norm - with the expectation that salary will be reviewed, and likely increased, once the probationary period ends and both parties agree that the employee is working out.
Some companies have regular, usually annual, reviews. Typically the past year's performance is measured and discussed, and salary increases are handed out. Usually, the entire company performs the Annual Review at the same time. In this scenario, new hires are most often give a pro-rated increase, based on the number of months they have been part of the company.
A few companies have annual reviews that are tied to the hire date of the employee. In this scenario, you would wait 1 full year from your hire to get your first raise.
And some companies have no set period at all. In order to get a raise you must ask for one. If this is the case in your company, you can approach the subject by casually asking your boss during a 1-on-1 meeting "So, what's the procedure for getting a raise at this company?" and taking it from there.
Other factors such as locale, union affiliation, etc - all go into when you can expect an opportunity for a raise.
Check all of these factors out in your context, learn what applies to you, then proceed. If you are still unsure, just ask your HR department, or your boss.