According to Nicole Grainger Marsh (Nicolegraingermarsh.com), most companies have a period set aside for assessing salaries. You just need to find out when it is. Most organisations have an annual salary review cycle that they adhere to, so your best bet is to make sure you get in and prime your boss ahead of time. This removes the common objection of, "It's not pay review time right now," Nicole says.
"If you have taken on any extra responsibilities in your role, and this looks set to continue, don't wait for salary review time to come around to have the conversation about you deserving increased pay. Any situation where you are delivering above and beyond for a sustained period of time, puts you in a good position to have the pay rise discussion."