if clients want a longer lasting eyebrow tint then they have to go darker then they want to to start with. it fades within a couple of days anyway. But because generally speaking you can only get tints in black brown grey and blue your limited to what you can offer them.
If i have clients that are hard to tint, i leave the tint on for 5 to 10 mins, explaining to them that this will giev them the best results as it will develop properly but may be a ittle dark at first, they are usualy fine with this.
also they MUST have it done regularly to get a build up of colour. if they come after maybe 2 weeks to start with they will have a decent build up of colour, so it should last even longer next time, so they may be able to move to 4 weeks. then they must get them tinted and waxed if they have that done, every 4 weeks, religiously. I promise, it works!