Confession: although I haven't had a regular job for five years, I don't balance my checkbook or keep a budget. I do know within about $100 what my checking account balance, my monthly income and expenses are--and so should you. You should know this before you stop working full-time, or working however much you are used to working. Financial planners say you should know these and several other financial facts about five years before retirement so that you can make any adjustments you'll need to make before it's too late to make them.
If your savings, pensions, and investments combined with Social Security benefits will not generate enough cash to cover your expenses in retirement, you might change your investment strategy to skew toward higher dividend producing stocks, and interest income from bonds. You might need to cut expenses or rethink whether you will work part-time at what you're doing now or something different, or rethink your whole strategy for the next phase of your life.
If you do start thinking about this several years before the change you plan to make, you have ample time to make adjustments. There are software programs and people who can help you with asset allocation, housing changes and costs, and other expenses. Once you've got this BIG piece figured out, the creative choices you make about travel, painting classes, golf lessons or square dancing groups are much more fun to think about and plan.