The duration is neither undefined nor null; it is 1d4+1 rounds (apparent time); see text.
Ok -
This spell seems to make time cease to flow for everyone but you. In fact, you speed up so greatly that all other creatures seem frozen, though they are actually still moving at their normal speeds. You are free to act for 1d4+1 rounds of apparent time. Normal and magical fire, cold, gas, and the like can still harm you. While the time stop is in effect, other creatures are invulnerable to your attacks and spells; you cannot target such creatures with any attack or spell. A spell that affects an area and has a duration longer than the remaining duration of the time stop have their normal effects on other creatures once the time stop ends. Most spellcasters use the additional time to improve their defenses, summon allies, or flee from combat.
You cannot move or harm items held, carried, or worn by a creature stuck in normal time, but you can affect any item that is not in another creature’s possession.
You are undetectable while time stop lasts. You cannot enter an area protected by an antimagic field while under the effect of time stop.
That is the "text" ... the bolded portion indicates that I am free to act for the
equivalent of 1d4+1 rounds. However, those rounds aren't "really" there. The spell is cast on ME. The rest of the world is still moving, but I am at "super-speed", so fast that it visually looks like everyone else is "frozen" in mid action. In the Normal time stream - how much "time" passes? It doesn't say.
Hmm, experiment!!!!
Required Materials:
6 Stopwatches, synchronized.
1 Time Stop Spell; Maximized.
1 Measuring string
Nearby Windmill in motion.
1. Set Stopwatch at base of windmill (stopwatch G)
- Cast Timestop (windmill will visually "stop moving") and Hustle
2. Have stopwatch on My person (Stopwatch A)
3. Have stopwatch set 30 feet away (Stopwatch B)
4. Have stopwatch set 60 feet away (Stopwatch C)
5. Have stopwatch set 90 feet away (Stopwatch D)
6. Have Stopwatch set 120 feet away (stopwatch E)
7. Have Stopwatch set 150 feet away (stopwatch F)
- Time Stop - End (windmill will visually "resume motion")
8. Pick up stopwatch set at base of windmill (Stopwatch G)
Cast TimeStop
Pick up watches in letter order, from A to G.
Record Difference in times shown, if any from stopwatch A to G.
What is the result of each stopwatch?