I always change my toothbrush with a new one every 3 months.
With daily brushing 3 times per day my toothbrushes get worn out pretty fast and toothbrushes are pretty cheap and I get them for $1.00 each at dollargeneral stores.
My dentist recommended I change my toothbrush every 3 to 4 months and so I change my toothbrush and get a new toothbrush every 3 months.
Also never share toothbrushes either because of germs and after you use your toothbrush rinse any leftover toothpaste and other debris off of the toothbrush before putting it away.
If you're rinse your toothbrush off before putting it away then there could be bacteria leftover on the toothbrush which is not good.
When you use your toothbrush the toothbrush bristles become frayed eventually which makes the toothbrush less effective at cleaning your teeth.