part of it has to do with your resistance to spending.
there is no set formula, like it has been 1000 days since i bought my last bike and the term of limitations is up, so i better start looking for a bike.
My girlfriend just got her huffy 3 speed fixed up, the bike is 40 years old. She's thrilled to have it working again, she loves it. And it will do what she needs it to do!

If you can afford it, go shop around. It's good for the economy. and unfortunately, shopping for bikes is REALLY thrilling, addictive and fun.