My preference is Refectocil. It's not the cheapest, also not the most expensive. You can get it from Beauty Express. It's hard to say how long it lasts because just like head hair, lashes vary enormously from client to client. Clients with thick porous lashes will probably have a longer lasting result than those with very fine lashes. I personally would encourage clients to have it done monthly, although some clients can go 2 months.