Even if the correct time is 4 minutes or 12 minutes, and you develop for 7 (or 5, or 10) minutes, you should still get printable negs. From inspection of the resulting negs you will quickly find the dev time most suitable for your dev setup (temperature, agitation regime, time needed for pouring liquids, ...), without losing any rolls.
There are very few developers that will give you completely over- or underdeveloped results with 10 minutes @ 20°C development, and those few are usually described and labeled as such ("tropical developer", "very low contrast developer", "for stand development", "very active developer").