How do I make my extensions last so long?".
Unfortunately each individual has their own lash cycle so there's no absolute way of making lashes last as long as the next persons.
For me though I don't touch my lashes too often. I make sure my eyes don't touch water for at least 48 hours after the initial treatment. After that I wash my face and shower as normal letting the water run through my lashes.
I think it's important that water goes through the lashes so it clears away any make-up, dirt, and dead skin cells. By not washing the lashes regularly, you're allowing the dirt and grime to build up making a nice environment for infections to occur.
Most cleansers, make-up removers, soaps, face washes contain an ingredient called "Glycol". Unfortunately this particular ingredient is an ingredient contained in the lash remover used to remove lash extensions. I recommend using cotton buds to remove any eye make-up around the eyes using either water or a water-based make-up remover like "Simple". Dab never swipe across the lashes as you may snag a lash - ouch!!