If it is a reasonable workload, I would charge a bit more for late notice. If it is clearly unreasonable workload or music that is too difficult to learn in a short time, I decline. I'm open to taking on emergency gigs, as long as the music is doable with reasonable expectations and the collaborator is willing to pay a serious premium over my normal fee.
There have been many times I've received a vague voicemail or e-mail of the following nature:
Soloist: My name is ____ my recital is next week. I have three pieces, are you available?
Me: can you provide copies of the music?
Soloist: Here are the 12 pieces on my recital (about 50 pages of material). There are another two pieces, but I'm not sure I want to sing them yet. I could only find copies of 8 of the pieces, I'll send the rest later. By the way two of my friends are doing a recital on the same day as me, can you accompany them too? BTW, can we rehearse tomorrow at 2 pm?
Me: Here is my fee for the services requested ____
Soloist: Why you charge so much? Can't I just pay you $20. Since my friends recital is on the same day, the $20 should be able to cover the cost of it too.
At this point I would decline.