You should take your mum to see the eye doctor every year. As to you, though you may not have any problems at this moment, you also can have your eyesight checked every two years. Some disease such as diabetes, macular degeneration, high blood pressure, or glaucoma will cause some vision problems. So we must be careful too.
Dear friend, though you have very good eyesight, I recommend you to have your eyesight checked yearly to watch for diseases. Some potential disease can cause real problems with your vision. Your mum must to have her eyesight checked regularly too. I think the time is a little shorter than one year. If she has very serious vision problems, she can have her eyesight checked every half a year. So the eye doctor can give her effective treatment.