I've had lash extensions for over 7 years. I go 3-4 weeks between infills although sometimes I could go longer.
I don't bother buying all the expensive lash shampoos! Buy a foaming pump bottle off amazon, get some johnsons baby shampoo and water in there. That's all you need! I use a soft brush to apply the shampoo mix, then I brush through with a lash spooly. I then get a dry lash spooly and put my hairdryer on the cold setting to dry them and keep them fluffy!
I wash my lashes about twice a week, sometimes more, sometimes less depending on how grubby I think they are. I don't wear any makeup round my eyes though so tbh they don't really get dirty!
Brush them daily, avoid any oil based products near them at all! try not to get them wet in the bath/shower and if you do then dry them as above with the cold setting on the hairdryer and they'll be fine!