Over the years, I've seen where there is a debate on whether you should charge by time for touch ups or by weeks.  For some artists, charging by weeks has worked out great and it's manageable.  For others, it's easier to charge by the time.  Personally, I've done both and currently, I charge for touch ups based on time.  Here's what I noticed before I made the switch.  

The most apparent issue I had when charging by weeks is that my Clients would push the limits and wouldn't come in for a touch up at the time I recommended.  Instead, they'd wait the longest amount of time possible and barely would have any lashes remaining.  I found I'd be performing a full set and taking more time to do so - but at the cost of a touch up.  I had a rule that they needed to have at least 50% remaining to be considered a fill but in all honesty, I never met a Client that was very good at knowing how much they had remaining.  

Also when I charged by weeks, I would have to go back and check my records to see how long it had been since my Client's last appointment.  Some Clients would swear I was wrong even though I had the records to prove otherwise.  Personally, I like to keep things really simple for myself and for my Clients.  So having to debate or look back at the calendar wasn't the best use of my time.  

Now I charge by time and it's made life so much easier.  I implemented an online booking system so my Clients could select the amount of time they felt they needed.  The only challenge with charging by time for me was that I had spoiled my Clients, giving them a ton of time without charging extra.  Because of that, it was difficult not going over the time they booked.  But I learned to stick to the time and as a result, I've found that my Clients do a better job caring for their lashes.  Miraculously, when people know they aren't going to get a full set at a touch up price, they manage to take better care of their lashes.

It's a good move as a professional because you have better control over your time and income.  Instead of being blindsided by a full set and be late for your next Client, now you know what to expect and can better manage your day.  

In conclusion, I recommend charging by time once you know how long it takes to apply lash extensions for touch ups.  If you're like me, you may need to list a few options broken down by the kind of lashes you're touching up.(classics or volume)  Since I made the switch, it's been quite positive.