How long did Miriam have to wait by the river before Basya bas Paro retrieved Moshe from the box?
"You shall neither prostrate yourself before them nor worship them, for I, the Lord, your God, am a zealous God, Who visits the iniquity of the fathers upon the sons, upon the third and the fourth generation of those who hate Me,and [I] perform loving kindness to thousands [of generations], to those who love Me and to those who keep My commandments."
From here we see (assuming "thousands" plural, is a minimum of two) that the measure of justice to punish is 4, but the measure of kindness for reward is at least 2000. This would mean that the ratio of reward to punishment is 500:1. This measure can be used to know that the kindness/reward shown by Hashem for the good that people do, is minimally 500 times the value of the good deed.
Numbers 12:15 "So Miriam was confined outside the camp for seven days, and the people did not travel until Miriam had entered."
Miriam was rewarded for waiting for Moshe, by having the Jewish camp wait for her an extra 7 days before travelling on. (see Rashi who brings Gem. Sotah 9b)
7 days divided by 500 = 20.16 minutes. Since she waited for about (at most) 20 minutes, the camp waited for her 7 days.
- Lubavitcher Rebbe, from Sicha of Shabbos 19th Shvat 5741 (1980)
(see also Tosfos Sotah 11a which really comments on the Mishnah on 9b, credit to Elcho; OP)