Assuming a linear rate of spending, you could obtain the percentage
spent so far (i.e. money spent divided by total amount), and then
multiply this by the overall period in which you have to spend the
money.

However, this makes a lot of assumptions. If you add the start date to
the overall period, then you will have the end date, and if you
subtract the function TODAY() from this it will give you the number of
days remaining (or however you format it).

Hope this helps.

Pete