How does a loan modification effect my credit? And for how long?
A loan modification in it's simplest form shouldn't not affect your credit in any way. With that being said, there are some scenarios where your credit could be affected.
For example, if you do not make your mortgage payment while negotiating the loan modification, this would obviously affect your credit in a adverse way.
If you are currently delinquent or have received a notice of default, which many people seem to wait for before they start exploring ways out, then this as well will affect your credit. If you have a foreclosure/NOD on your credit, you probably will not be able to get another mortgage loan for at least 3 years, depending on the lender/program.