After over 20 years as a working professional, I can tell you that there is only one consistent rule with regard to raises: If you don't ask, you don't get.
Otherwise it's all relative. You have to consider the following:
- Are you adding sufficient additional value to the company?
- Is the increase within what that market expects for someone with your job?
- Would it be difficult to hire someone else to do your job?
And most importantly
- Can you easily quit this job and find another?
For example, in my line of work, it could potentially cost an employer $50K (or more) to find someone to replace me, since it takes a new employee about three months to get up to speed. Knowing this, and knowing the market, I have a good idea how much more I could get if I did negotiate a raise.
Of course you have to know your boss and the overall company culture. Some times they recognize they're getting a great deal with you and they should pay you more. Some times you have no choice but to strong arm them by threatening to leave if they don't pay you what you think you're worth.
But as I said, without taking the initiative, you'll never know. In your particular case I would take the tactic that you're doing a lot more work than you were told you would have to do when you got hired, so you deserve more, and possibly that you feel unappreciated since you've been given more work with no increase in compensation.
They will probably respond something to the effect that everyone is working harder, and you're the only one complaining, so why not be like all the others. To which you respond that you can't be responsible for anyone else's career choices, only your own.
At the end of the day, one of three things will happen:
- They will offer a sufficient raise, and you will be happy.
- They will refuse to give you a raise, and you will accept this and continue to work
- They will refuse to give you a raise, and you quit.
or you never ask, and you never find out what would have happened.