How and when should I buy an iPhone

Hi guys, I am currently owning a Samsung S24 plus 512GB variant. Lately, I started looking to buy myself an iPhone (either 17 or 18). Not sure, whether it is a proper or right way to upgrade

Could you guys please enlighten me how do I upgrade to iPhone and down the line, how many months should I be waiting and getting a new one.

Fyi - my phone is 3 yrs old