Well you certainly had a challenge in yesterday's temperatures! If the lashes don't last as long as you'd expect, don't blame yourself as the glue doesn't set as well in this heat, so even if you managed to keep your room cool, as soon as the client left you she woukd have been exposing them to extreme temperatures.
My daughter contacted all her lash clients before their appointments yesterday warning them that they might not last and if they still wanted them doing she got them to sign a disclaimer!
With regards to how long it took her to feel confident doing lashes, she actually went on another training course as she just didn't feel happy with the lashes she was producing after her first course. She did a lot of research to find the right trainer and then did a 1-2-1 day with her. It literally all clicked into place after that session and she's been fully booked ever since and has loads of regulars. (Shes also trained to do Volume lashes as well now). It's like most things, the more you do, the better you get I think. She did a one week special offer straight after her course and just did one set after another (she's never had any trouble getting clients) and that was what she needed.