Like Scrappile said, the first give away to a dull blade is having to push a little harder to cut. Don't let that fool ya with tension. If your tension is to loose to begin with, your blade will wander like a drunk in the street!
In turn, dulling your blade almost immediately. Depending on what wood you're cutting, the thickness of it, & the speed of the blade all play into how long your blade will actually last. Speed creates heat, so the faster you cut, the heat will take the temper out of the blade causing it to dull & burn the wood. You'll be able to smell a light burning wood scent if you're paying attention.
Wood species will dull a blade pretty fast, too. The Harder the wood, the faster the blade will dull. Cherry, oak, walnut, maple, just to name a few, will not eat a blade as fast as ash, teak, cypress, etc. Cherry & oak wood WILL burn if your blade speed is to fast, in turn, dulling your blade.
The thickness of the wood you're cutting will dull blades, too. Make sure you're using the right size blade for the thickness of material you're cutting. The thicker the material, the less teeth/inch you want, & a little less tension. The thinner the material you're cutting, the MORE teeth/inch you want on the blade, & a little MORE tension. Every blade makes its own sound when cutting. As a rule, if your blade starts to make a squeal sound, it's done. Don't force it to cut, you'll just bust the blade.
And as Scrap pile also mentioned, try several breeds of blades in your projects til you find what works for you. Personally, I keep 30 different types & sizes of blades of Flying Dutchman & Olson. With more experience, you'll soon figure out every project, unless it's a small one, usually uses more than one size & type of blade. Keep practicin', & keep askin' questions. We were all beginners at one time
Enjoy the day!