Not doing love spells with a particular person in mind is a Wiccan concept, and not everyone is Wiccan here
Some people, when they work money spells, work only for the amount that they need and some, like Michael said, don't set limits - so do what you feel comfortable with.
When they should be done? --Whenever you need money. I would say best worked on the waxing moon but if you're desperate work the spell anyway or try for a Sunday or if you can't do that then the hour of the Sun.
Be sure it's money you need to work for - for example, if you're in a low paying job, instead of doing a spell for more money you might want to cast a spell for a higher paying job.