A crossbow can last many years and at least tens of thousands of shots, provided you maintain it properly and replace the parts when they wear out - mainly string, cables, and even limbs. In that sense, you should treat those parts of your crossbow as consumables, because this is what they are, in practice.
When aiming to prolong the life of a crossbow and its replaceable components, you should realize that both keeping it loaded and shooting it puts a strain on those components and ultimately damages them.
Yes, you will postpone the moment when you need to buy a new string or a serving by doing regular maintenance, but you can't use them infinitely. So be prepared to spend some money on renewing the worn-out parts.
First, let's talk about...
Some of the most recognizable crossbow brands like Excalibur and TenPoint offer a lifetime warranty on manufacturer defects. The "lifetime" part refers to the lifetime of the original buyer.
Of course, the warranty does not cover damages to the product that is the result of misuse or abuse. An example of such misuse is dry-firing the crossbow - for many manufacturers, such action would void the warranty.
Also, certain crossbow accessories are categorized as "normal wear items" and thus not covered under the lifetime warranty. Such items are:
Some manufacturers warrant certain parts for a set number of years after purchase. For example, TenPoint offers 5-year warranty on the limbs, scope, and the cocking device.
Here's a comparison table of the product warranties per manufacturer:
If you read through the warranty text on each company website you should be able to get a pretty good idea of what sort of longevity to expect from their products. For instance, TenPoint's 5-year warranty on limbs is a useful "hint".
But even if the warranty is not that important to you at the moment, you might want to stick to a company that is not likely to go out of business. If the longevity of your crossbow is a priority that is.
Just think about it for a moment - if you will be using it actively, you'd better plan in advance replacing those parts. Note that the bowstring should be replaced every 3 years whether you shoot the crossbow or it just sits in the closet.
Now imagine if the company that made it stops producing parts for your model...
And despite the increasing popularity of crossbow hunting, that is not that uncommon. For example, two prominent archery manufacturers - Parker and Bear, went out of the crossbow business during the last couple of years.
In this regard, it's probably a good idea before buying a crossbow, to check the availability of accessories for that particular model. Just browse Cabela's or your favorite archery store and see if you can find the items that are compatible with your chosen model.
Of course, it is assumed that you are informed of what consumables and replacements parts are best for the crossbow model that caught your eye even before purchasing it.
Another variable with direct impact over the "health" of a crossbow is the arrows you shoot through it. Every model has a recommended set of parameters for the arrows. The material the arrow is made of, its weight, its vane length, its nock shape - everything should be in accordance with the manufacturer's recommendations.
The largest crossbow companies actually produce arrows that are made specifically for their crossbow models.
Using inappropriate ammunition would not only affect negatively the performance of the weapon but is also very likely to damage or even destroy it.
Just an example: a secondary function of an arrow is absorbing the shot energy and reducing the force and the vibration the string and limbs suffer each time you pull the trigger. So an arrow that is lighter than the recommended standard would not provide the needed protection.
And finally, take care to not leave the crossbow cocked for too long, clean it regularly and store it in a safe place - away from children, dust, rain, snow, and sunlight.