@Andrew FazzoneMy very short answer is "Immediately! NOW! Today!" Maybe that's 3 answers, but pick whichever you like best.
If for some reason you are ignoring the above answer and still reading, here's a longer version. I am assuming your definition of "investment properties" is buy-and-hold. I also assume we are talking about the Louisville market. Given this, there are 3 critical aspects to my answer.
1) We had little to no appreciation from 2008-2014. Even last year was fairly modest. As a result, the gap between new construction prices and existing inventory prices is huge. Until that narrows, demand for existing inventory will be higher. This will force appreciation until builders can profitably get back in the game. Buying today will allow you to get in before prices creep up.
2) Interest rates are still at a historic low. A 1% increase drops your buying power by nearly 10 percent. As an example, a $110,000 rental property financed (20% down) at 5% over 25 years has a PI payment of $514.44. If interest rates go up by 1% and you want to keep the same payment, you would have to buy that same house for $100,000.
To not be buying today, you need to believe that either prices are going to drop or interest rates are going to drop. I don't think either of those options are true. Additionally, this is a long-term strategy. During the next 10, 20, or 30 years we will see positive and negative market fluctuations. You can't wait until conditions are absolutely perfect to jump in. (That being said, I think we are pretty damn close to perfect).
More importantly...
3) The numbers work. This may be the only thing that really matters. For $110,000 you can get a completely updated house with a new roof, new HVAC, new water heater, new kitchen, etc. And, you can rent it for $1000+. After taxes, ins, and PM you still cashflow (>$220/month) very well and you don't have to do anything. You can find this deal all day long. Obviously, if you get a 4%/30-year loan or if you want to trade your time for dollars, you can do better.
In conclusion, GO BUY A HOUSE!
Regards,
Erik