Do caps lose their "strength" over time? Do they have a limited life span and do we need to swap them out for new ones every so often?

They have limited shelf life of around 5 years, This also depends on stress etc. They also fail fast with extreme temperatures, but you will break it by crashing way before anyway
I wouldn't worry about it in a quad You will probably replace it way before it becomes bad.

(16-Dec-2018, 11:30 AM)beastmode92 Wrote: They have limited shelf life of around 5 years, This also depends on stress etc. They also fail fast with extreme temperatures, but you will break it by crashing way before anyway
I wouldn't worry about it in a quad You will probably replace it way before it becomes bad.

And the good thing about electrolytic caps, they usually let you know when they are done by leaking before they fail hard.  

Beastmode92 is correct that one of the biggest reasons they fail is how badly they are stressed.  You can extend their life if you use them with a rated voltage much higher than you need.  For example use a 50v cap on a 6S system rather than a 35v cap on 6S.  If you don't stress them, they can last a very long time.

They can last for a long time before they fail. Old CRT TV have a bunch of 400-500V capacitors (which I used to make stun-guns from when I was little ) and those old TVs can last for decades..

The important thing with capacitors is not to cheap out on them, NEVER buy an unknown brand and NEVER buy from a Chinese seller unless you know for certain that the item you are buying is 100% a genuine branded product. If in doubt, just pay a a bit more and buy from a reputable source in your own country.

The highest quality capacitors are from the following brands:-

• Panasonic
  
• Rubycon
  
• Nichicon
  
• Chemi-Con
  

I would steer clear of anything else or any other "similar" Japanese sounding brand names that are likely purposely named that way to confuse you into thinking they are a top brand to try and make you buy them. I generally buy Panasonic capacitors because it's also a brand I can relate to in the consumer electronics industry, but any of the above brands are manufacture top quality capacitors.