A soft bristled toothbrush for a person brushing twice daily should work for 3-4 months. (I don't recommend a medium bristled and hard is complete no-no)
If you are changing sooner than that because the bristles have frayed, then you are brushing way too hard.
I normally use my toothbrush for about 2 months before I chuck it for a new one, although if the bristles go too soft for my comfort, then I chuck it sooner - that's a sign of the brush not working as efficiently anymore if I remember reading this bit of information correctly :-)
My dentist said you're supposed to change plastic toothbrushes quarterly/at the start of each season. It's not just a marketing thing. The bristles get worn out and a lot of bacteria builds up on the brush over time. We use compostible bamboo ones so we change them every month because they don't last as long.
I'm a bit of a freak when it comes to changing mine and my families tooth brushes. I would say I easily change them once a month. I'm a big tooth brusher too, brushing my teeth quite a few times though out the day. And when I think of all the things we eat I think that we need something fresh after all that cleaning every day. It's kind of similar to the dish scourer and cloth wipes. I change them regularly too. I think I watched a segment on germs etc one day which made me a bit paranoid. So, I don't bother buying expensive tooth brushes, and always have a new packet of them handy.
I use a soft brush and change it at the change of seasons. So every 3 months. If I have been unwell, or have a mouth ulcer, I change my toothbrush to avoid infection.
My toothbrush is protected from bacteria by being kept in a cabinet away from accidental splashes, being able to dry between uses and having the occassional sun bath.
I would love to stop using plastic toothbrushes, so will try to find the bamboo type Jennifer Muirhead mentioned.