Honestly, it feels a bit too D&D/vatican for a world. The exact "you can have X spirits" and "the more spirits you have the more the decrease in this manner" is so very...exact. In real life there is variation, one person can run longer then another, or some are better at multi tasking then others etc. I would build in allot more wiggle room for variation amongst people. I would also better explain why someone can only bind spirits in such a way, what limits them from having more bound or not releasing them etc?
Also, if a mage has to spend effort binding spirits, and can get as much out of one spirit as he can out of 8, why bind 8. Yes versatility is nice, but laziness is common in humans as well :P.
Thus a few things I would say, to keep with the general principle but better explaining why it exists and add wiggle room:
1) A mage can bind one/more spirits as long as they are able to, though it's nearly impossible to keep the mental focus required to keep a spirit bound when sleeping, thus spirits often are not bound for more then a day.
2) actively using a spirit tends to be draining on a mages power reserves. Using multiple spirits tends to exhaust them faster for obvious reasons, thus the reason that using 8 spirits generally tends to result in only being able to use them for 1/8 as long. However, strong mages may be able to do this for longer, it's not as exact as 1/8 as long for 8 spirits and mages and their spirits vary.
2b) weaker mages tend to only bind a few spirits, because they lack energy to support multiple. as mages get stronger they tend to bind more because while they can't keep a spirit bound for longer then a day they may have the energy to use a spirit for a day and use another bound spirit for awhile if they have stronger control.
3) spirits are 'competitive' in nature. While individual spirits tend to not care about a spirit of another domain they tend to react strongly to one of the same domain. Thus it's very hard to bind two spirits of the same domain (good or bad), and almost never worth doing instead of binding a spirit of another domain.
4) It is possible, in theory, to bind spirits again after the first binding in the morning, if you expanded all your bound spirits. However, to bind new spirits you must first release all your current spirits; you can't keep the 'lock' on bound spirits while drawing in new ones. It's also exhausting, magically, to bind spirits. Thus binding a new spirit during the middle of the day isn't common because they will have to do a magically exhausting binding again during the day, after already exhausting some of their energy using a bound spirit, and generally it's too exhausting to be worth doing.
5) You probably want to make sure your spirits aren't sentient, or else you get into lots of moral ethics about binding them. If they aren't sentient then the idea of a "good" spirit or "evil" spirit is meaningless, non sentient beings can't have ethics. In addition, it seems odd to have exactly two classes of spirits, but no more; no morally-neutral spirits etc. I notice, however, that your 'good' spirits always affect you and your 'evil' spirits always affect others. I would consider changing it to be that they only bind one type of spirit, but they bind it in one of two ways, to affect themselves, or affect others.
5b) I would suggest that the two classes of spirits (good/evil) are simply the most common/easiest ways of binding spirits. Other binding options exist, but people tend to stick with what is easiest and proven to work. Thus the list is not an exhaustive, and oddly exact, list of what can be done; just the common practical uses. Innovative, powerful, or just unusual individuals may bind spirits in different manner.
5c) the spirits you listed are not the only spirits in the world, just the most commonly bound.
