Should I get a new model of iMac?

Welcome!

And, I think average 5 years for each model of iMac before it turns to vintage model, right?

Not right.

Details of "vintage" are in this Apple support document:

Vintage and obsolete products - Apple Support

The criterion is not five years old--it is five years after the product is no longer sold. Some models stay in the line for longer than a year. Examples:

• iMacs do not always get upgraded every year. There were no 2016 or 2018 iMacs.
• The 2012 13-inch non-Retina MacBook Pro remained in the line unchanged until Oct 2016. Its status today is still "supported" for parts and service availability until Sept 2021.
• The Late 2014 Mac Mini stayed in production until Oct 2018

The free MacTacker database in the Mac App store has dates discontinued for practically everything Apple has made. If dates concern you I recommend you download it. It also has benchmark data.

Buy what you need now. If you always wait, you will never have what your need.

I do something different: I get Applecare which covers it for 3 years and sell it shortly before Applecare ends. Makes it easier to sell because the buyer has a month or so of full warranty coverage which gives them peace of mind.

It is totally up to you.

Personally I keep Mac my main Mac for 6 to 8 years and then repurpose the older one for family members or use it as a media server.