Rather than suggesting a specific timeframe to replace your toothbrushes, we suggest replacing them when they show signs of wear (which can be different for each person, depending on their brushing habits).
When the brushes lose their designed shape, they cannot perform the functions we want them to. So, we suggest replacing a toothbrush when its bristles begin to fray and frazzle and lose their shape.
Having said that, no matter which toothbrush you use, you really should get at least a couple to a few months of use out of it before the bristles start to bend out of shape. If you notice that your bristles are bending out of shape quicker than that, that can be an indicator that you may be brushing too hard. Here’s a link to an article that explains the importance of conscious brushing, here’s a link to an article and video that explains how to do the Bass Brushing Technique along your gumline, and here’s a link to a video that describes how to circle back and also brush your teeth.