What is the life span of a motorcycle?

Life span of a Motor Cycles or any Motor Vehicles are considered 15 years as per the Motor Vehicle Act, Transport Department, Government of India. Indeed, they have made this rules after getting all the technical details of/about a motor cycle or motor vehicles.
But this is also true that, When we get any new Bikes or Motor Vehicle we love it and enjoy it for few years only. So if you're planning for new one you can sell the old and go for the new one. Go for Bajaj Avenger Street 220.

Here are some of the answers to your queries.

1.What is the life span of a motorcycle?
Two factors here - manufacturer supported lifespan and consumer handling. Most of the motor manufacturers have the life-cycle for the product with spare-part support cycle of 10 years. Extended spare support is available through third party companies. For example there are Kinetic Honda, Bajaj M80 being still supported with sparepart using third party companies who make some of such spares. But officially companies only keep the product in same range supported not more than 10 years. As for the consumer lifespan that depends a lot on how the vehicle is handled. If used in rough road and tough conditions, then don't expect it for more than 5-8 years.

Is this the right time to buy a new bike? My bike has been maintained properly, both tyres have been changed in 2015 and it is running without any major problems.Should I buy a new bike or can I wait for some more time?

If after around 8 years the bike is performing well then you can either continue upto 10 years. I'd say keep it and then keep your eye on new bike so that you can switch as soon as the current bike starts having issues.

If I want to buy a new bike, which one should I buy? I like Honda Hornet, Suzuki Gixxer and Bajaj Avenger Street 220.I like the Avenger because of unmatched comfort and looks, but I was told that it is good only in a straight line and is prone to skidding.

This is subjective questions so answer is going to be subjective as well. i'd say Honda Hornet and Avenger both are good looking and also comfortable bikes. I didn't find Gixxer much attractive. Depending on the region where you wish to drive them, choose the bike accordingly. If you live in region where the roads are rough then avoid choosing bikes based on look.

The actual lifespan of a motorcycle will depend on how you maintain it. A properly maintained bike should last the lifetime of the owner. I have seen people using bikes for more than 20 years or so. However, the manufacturer may not support the bike for that period. You may have to source the spare parts from the third party providers.

As for buying a new bike, it will depend on the condition of your bike right now. If it has not crossed 10 to 15 years, it may not be good idea to change it. However, if you are fascinated with some of the new bikes out now, you may consider exchanging a new one. If your bike has been maintained well, you will end up getting a good resale value.

As far as your query on whether you should opt for any of the bikes you have shortlisted, I would suggest you to go through the reviews in authority sites. As long as the reviews I have observed, Honda Hornet and Avenger seem to have garnered some good opinions. Pay more attention to the features and sturdiness than the looks.