Я предполагаю, что, возможно, здесь может быть меньше границ,
(?i)\breminder\s+\d+\s+\b(?:seconds?|minutes?|hours?|days?|weeks?|months?|years?)\b
, а может и нет.
Тест
import re
expression = r"(?i)\breminder\s+\d+\s+\b(?:seconds?|minutes?|hours?|days?|weeks?|months?|years?)\b"
string = """
Reminder 3 seconds some data here, Reminder 20 days and some more data, Reminder 3 second and Reminder 3 WEek
"""
print(re.findall(expression, string))
Выход
['Reminder 3 seconds', 'Reminder 20 days', 'Reminder 3 second', 'Reminder 3 WEek']