After 6 months to a year you should get a credit limit increase as long as your credit card account is in good standing and you've never missed payments.
Sometimes you have to ask for a credit limit increase but some will automatically increase your credit limit on their own.
It's best to let the credit card company increase your credit limit without you asking. But it is okay to ask the credit card company for a credit line increase if you need or want the increase and are sure you can pay back the amount owed.