It seems to me that letting charitable organizations hold on to money for later purchases (over-funding them) just decreases their efficiency (amount of good done divided by amount of money donated). So a potential metric is whether the efficiency of a charity is going up or down over time (after adjusting for differences in the methods used to calculate the efficiency). If a charity's efficiency is increasing over time then it is under-funded. If efficiency is decreasing over time then the charity is potentially over-funded (but may just be investing in increasing it's productive capacity to do good). Once a charity's efficiency drops below that of the next-most-efficient charity, it is probably over-funded.

Additionally, money held in an account by a charity for more than a year can probably be personally invested for a higher expected rate of return than the charity is likely to feel is safe. If the charity can believe your precommitment then pledging to donate a lot of money (sufficiently "a lot" so as to over-fund them into inefficiency for several months or years) is probably more efficient than donating hastily. Set up a trust with solid investments and a reasonable payout schedule or something.

In personal terms, give what you can. That means balancing the opportunity cost of saving/improving random human lives against your own needs, which is more of a personal decision. A loan may be an appropriate donation if the rate of return is greater than the interest rate, but I wouldn't recommend getting a loan that costs more per month to service than you would otherwise willingly donate per month directly or getting a loan for more than the total donations a charity is expecting that year.