Much depends on your area, type of hardwoods you have... I have 100 acres of forest (family prop, didn't buy as investment) and here in the Atlantic/southeast region we have timbered it about every 30-35 years... Expenses are pretty straightforward (cutting in the roads, transport, etc.) just be careful not to run into any disputes with adjacent prop owners- be sure to leave some buffer and make that clear to the loggers...
I probably wouldn't look to buy land as an investment based on the timber value (unless you do that sort of thing professionally), but if it is land you want/own anyway, it's a nice little cash infusion... If you will be around in 30 years or plan to leave it to family, you can control the re growth somewhat and encourage Cherry or other high demand species... But no way to really know what will be high demand 30 years out, so it's still a gamble (but cherry, walnut, etc, are historically pretty safe bets)...