It's possible to spend either too little or too much time on strategic planning. Strategy is what you use to steer your company, so it's worth spending the right amount of time on. Strategic Planning is not, however, a substitute for the things that actually make your company go.
Far too many companies do strategic planning as a 2-day retreat. I imagine this has been driven by well-meaning, but amateur, "facilitators" who see strategic planning as an easy way to sell a weekend gig in between their "more important" work. In my experience - which is considerable - you need three meetings for a good strategic planning process. Each of the meetings asks a different question:
1. Where are we?
2. Where do we want to go?
3. How will we get there?
Most people pretend you can just ask the middle question. Sadly, as in any navigation, if you don't know where you are, you can't really figure out the proper direction you should be going. Worse yet, in strategic planning, if your management team doesn't AGREE on where you are, they won't agree on your course. You'll save yourself a lot of headaches by having a meeting before you strategize, to figure out what you need to know and how to structure that information. If you skip that meeting, you will likely end up with poor strategy - or, at best, a poor discussion of your strategy.
The third meeting is just as important, because it's not enough just to have a strategic plan. You have to implement the plan, and the hundreds of plans we have completed have shown conclusively that an implementation framework (and a few other tricks we use) greatly increases the number of strategic objectives achieved. In other words, with an implementation plan, you will actually end up doing most of what's in your strategic plan. Without it, you are likely to achieve only 30% of your objectives.
The last comment I'll make on spending time on strategic planning is about spending too much time on planning. Your team has work to do, and planning is only a part of that work. Four or five meetings are not better than three, and if you try to do your planning in, say, an hour a week, you will never, ever get through the process. Almost everyone should allocate between four to seven days (that's 8 hour days) for their strategic planning process every cycle - and maybe another 10-20 hours for homework. That's it. If you need more time, you are probably attempting to implement your objectives inside your strategic planning meetings. While this is admirable, in some respects, it will ultimately sink your strategic planning. So make sure you set a practical, realistic schedule for strategic planning - and stick with it. Otherwise, it can eat you alive.
Naturally, a good, experienced strategic planning professional will tell you these things, and help you navigate all of the questions that come up about the strategic planning schedule.