I would have to say that depends on what you are doing. If you just keep pumping the same thing on someone over and over, I would say stop and give a chance to work. If you are doing a lot of different spells on someone, I would say that you may want to look at source of help other than magic. It's not the only tool in the tool box.
Medtation, energy work and basic ritual, in that order. I recommend Silva Life Systems (http://www.silvalifesystem.com/free-lessons) for meditation and some form of Chi Kung (www.taichi18.com) for energy work. Basic ritual is going to depend on the tradition you choose to follow. It could be the LBRP, opening of a circle, honoring the gods, etc.
Do the meditation training first. When you have finished that do the Chi Kung. While you are doing these, start to research different traditions. When you find the one you want to follow, it will have basic ritual work for you to do AFTER you finish studying Chi Kung.
regarding your first question of casting too many spells, yes and no. you can cast too many spells, this can leave you drained, possibly make you sick. spells need time to work, and your body needs time to get more energy. wait a while between casting. if you're casting over a person for the same thing, i personally wait one lunar month before casting another spell for the same thing. if you're casting a luck spell, health spell and a money spell on someone, i'd cast the most important one first, then wait a few days [maybe a week] before casting the next spell. if i felt up to it i might cast more than one spell, but spells take a lot of energy. when i know there's spell casting [or my coven's using the Ouija Board] i keep sweets near by otherwise a lot of my coven members energy crashes and they tend to fall asleep. just figure out your limit, eventually you'll be draining yourself and your spells will be pointless.
So the question here is about repeatedly casting spells over the same intention.
It can be certainly done successfully, while casting for the same intent or desire only once may be as effective and egregious.
The amount of spells cast upon the same eagerness or penchant aren't very substantial as long as the first intercession of spell casting was efflorescent / proliferating.
If your exhoration for multipate casting is keeping the spell and its outcome to be held successfully longer, exclusively holding up the energy built up for the spell casting and resending it can be as efficacious and advantageous for keeping the spell's outcomes for a longer period of time.