وَيَجْلِسَ بَيْنَ كُلِّ تَرْوِيحَتَيْنِ مِقْدَارَ تَرْوِيحَةٍ.
Translation: And between each two tarwihahs, sit for a period approximately equal to one tarwihah.
The interval is roughly the time of one four-rakah tarwihah. It is a recommended organizational practice, not a fixed obligatory duration whose omission invalidates prayer.
There are normally four such intervals between five groups.