Unfortunately there's always going to be clients who try to "test" your refill policy boundaries. Because of this it's super important to be very clear from the beginning what constitutes a refill and what doesn't.
For starters, if you're charging refills by "weeks" you need to switch to charging by "time", this is the best way to manage client expectations to ensure they leave happy.
If a full set takes you 1.5 hours to complete and your client continually arrives to their 3 week refill (that you've only set aside an hour for) with only a few extensions on, you're going to be stressed and your client will probably leave dissatisfied as her lashes won't be as full as when you applied her original full set.
Not a good experience for anyone.
Our @brisbanelashes refill policy states you must come in for your refill within 3 weeks of your last appointment AND have at least 50% of your extensions remaining to qualify. This also encourages clients to take better care of their lashes through daily cleansing, using lash friendly makeup and no picking etc.
If a client books a shorter refill appointment than what they need (knowingly or unknowingly) we always advise upfront BEFORE starting that we will not have enough time to complete the full refill and will focus on the outer 3/4.
For their next appointment we would advise to either come in sooner, or to book in for a longer refill appointment to ensure we have enough time to complete the service.
By doing this you're not putting yourself under unnecessary pressure to perform a miracle and the client won't be underwhelmed when she opens her eyes at the end.
Remember, every client has a different lash growth cycle and some are more particular than others about the fullness. Because of this we have some clients who prefer to come in every 1.5 weeks so not a single lash is out of place, whilst others are happy to come in every 3 weeks.
Know your clients and be upfront and clear about everything you do is the key to a good client relationship.