Basically, when either the amount of money is very large (compared to what they already have) or all other motivations cancel out.
Human beings are motivated by many things: Emotions such as pleasure, love, loyalty, anger, revenge, and hope. One of the strongest is groups they identify with such as families, countries, religions, or even sports teams. For some people, these motivations can become so strong as to outweigh anything else--even their own survival.
However, other people are not strongly motivated by these things, or live with them in good enough balance that they don't have to do much to satisfy them. In those circumstances, money can be a very powerful motivator. The fundamental basis of a capitalist economy is essentially the assumption that money can motivate people to do useful jobs that they would otherwise prefer not to do; and so far, capitalism seems to work better than pretty much everything else.
Another important consideration is how much money you already have; $100 is much more motivating to a starving child in Burkina Faso than it is to a middle-class office worker in Ohio, and in turn more motivating to the office worker than to a billionaire CEO. To the starving child, it is months of income, and possibly life itself; to the office worker, it is a night out with a fancy dinner and a movie; to the billionaire, it is a rounding error on a spreadsheet. We call this diminishing marginal utility of wealth--the more wealth you have, the less you gain from each new dollar.