My first set (from cosmetics to go in the 90s) lasted me 10 years, then upgraded to gap (& odd laura mercier/bobbi brown). They last makeup artists 5-10 years, so expect mine to last me rest of my life...
they do harbour bacteria though...so as a spotty teen i made sure to cleanse mine.
Mac do a makeup brush cleanser for £8 which is overpriced, but you could use once in a while (esp good for foundation/gel eyeliner/cream eyeshadow which can be difficult to remove otherwise), or sterilise with alcohol. I wash mine every few weeks ... with shampoo, handwash or facewash
My mac cleanser has run out after 2 years so now planning to spritz the washed brushes with alcohol (spray bottle of vodka, also good for deo stains on armpits of clothing apparently) and let dry on a towel.
The main way to keep the brushes going is to NEVER soak them: if really dirty mix shampoo in your hand & massage in before rinsing
I would not buy special products to clean, but the most effective from memory are S'by white hand wash, & Neutrogena pink grapefruit
I know far too much about this .. makeup artist in the family!