Toothbrush must be changed often and this is very important to keep up overall hygiene!
It's always best to change to a new toothbrush every three to four months at the very latest to not open up the mouth and the rest of the body to germs, diseases and so on. Apart from being able to 'tell' a toothbrush needs to be changed (from the obvious worn out bristles in some cases) regularly.
When a toothbrush is used continuously without changing-three to four months per toothbrush even where you use a spinbrush/electric brush you should change the detachable head often- a lot of horrible things can happen/go wrong and these ones top the list:
1. The risk of Ingesting dirt/bacteria
with toothbrush there's always the possibility of ingesting dirts and germs which can accumulate over it over time. Toothbrushes are sometimes kept away moist leaving room for germs/bacteria to breed on them and a continuous use of this is harmful to the body at the end of the day.
2. Risk of gum damage
Using a toothbrush for a prolonged period get the bristles worn out which can in turn damage the delicate gum/gum area/erode the enamel which is a perfect way to harbour infections which left untreated can be very harmful to overall health.
3. Bad Breath
Using toothbrush for longer than necessary opens it up to different unpleasant smells which can be transferred back to the mouth and the least this would result to is bad breath apart from other obvious serious issues.