How much to spend for an extra $100 in rent? ROI

[Dan D.]

When updating a property for a rental, what do you feel is a reasonable time frame to get back your improvement investment.

Example:

If I have a property that I can rent for $1,000 a month, but by spending a bit more, I could raise the rent to $1,100 per month, what amount is reasonable to spend to get that extra $100 per month ($1,000-$1,200 per year) of extra income?

If I spend an extra $1,000 for a $100 per month increase, within 12 months I should have my investment back.  What do most of you do?