Redeem means "to buy up for oneself" or "buy up an opportunity." When connected to "time," it means "to buy or take advantage of an opportunity." Since we are dealing with time, and it inexorably passes on, we must make the most of each opportunity. If an opportunity is missed, it cannot be recalled.
Paul might as well be saying, "God's way is not just for a few hours on a Sabbath, but the will of the Lord applies in every situation in life." He is urging us to take advantage of every situation to imitate God. Every second of our lives is precious in the building of godly character.
This has nothing to do with literally gaining time. It may be illustrated, though, with the example of the savvy merchant who takes advantage of every opportunity to make a profit. Businessmen often say, "Strike while the iron is hot." Or, in our consumer culture, we watch the advertisements to take advantage of a sale.
How can we determine how to take advantage of time? In much the same way we take advantage of a sale. We decide what we want and then we watch carefully. If we want to buy a product, we will generally survey the market and then decide which particular brand and model we want to buy. Then we keep a careful watch until the sale of our choice occurs.
Likewise, we must survey what is controlling our time and we must decide what is important to us. Is it our relationship with God, family, job, socializing, recreation, or entertainments? In what order of priority would we put these and other interests? Next, we must survey each of the major categories more specifically and insert particular activities into them. This is very important because the "small" activities drain most of our time away almost unnoticed.