All children are different. I have two, a girl and a boy, and they both learned to use the potty at different ages. My daughter didn't stay dry at night for a long time after she'd mastered use of the potty during the day, while my son was dry at night before he started using the potty. To an extent, staying dry at night is physiological and not something you can do much about if your toddler won't wake up to go pee at night, the obvious solution is a night-time diaper.
But, that doesn't mean there are not steps you can undertake to encourage staying dry at night while she is working on potty training. The things you can do to help your toddler stay dry at night include:Encouraging your toddler to use the potty right before going to bed. Not allowing your toddler to drink gallons of water or juice before bed, which doesn't mean you should withholddrinks when your child is thirsty. Some parents even take their toddler to the potty in the middle of the night. I've never done this and don't like the thought of waking my little one up to go pee, but some other mothers have done this successfully with no protests from their kids.
Then, there are things you can do to minimize the damage when your toddler does pee himself at night, something that is almost inevitable at some point. A night-time diaper is good while your child wants to wear one. When he is already mostly dry during the night, you can instead try a mattress protector. This will keep your toddler's bed from becoming smelly, and still makes your child feel like a big kid who doesn't need diapers anymore. If night-time wetting goes on for a long time, do mention it to your child's pediatrician to rule out any problems.