Depends on the application and what you feel comfortable with. I have rods ranging from 4.5 feet to 16 feet. I troll using rods from 9 feet to 16 feet. I have seen some people troll using only 7 footers because that is what they like. When I am fishing minnows with a cork or pitching jigs then I usually don't go any longer than 7 foot, but will sometimes use the 9 footer to reach in amongst a tree top without bumping the branches with the boat. I use my 4.5 footers a lot for pitching jigs and spinner baits. Just use what is comfortable for your style of fishing. When I first started out I primarily use rods in the 6.5 to 7 foot range. When I started experimenting with other techniques I started to buy more rods in different lengths. I don't have the top of the line equipment, but I have what will fit my budget and my style of fishing, which will vary depending on the time of year and what the fish are doing.
Hope I was able to shed some light on this for you. There are a lot of experts on this board who will be more than happy to share some of their insight with you on this topic.