"It took twelve months."
Someone had explained but I lost the thread. (Another problem of Duo.)
This is a little Ke symbol but it is pronounced as が. It is a counter word for month. 十二が月 is 12 months while 十二月 is December.
【が】 when used as a conjunction, 【か】 when used as a counter. Source: https://en.m.wikipedia.org/wiki/Small_ke?oldid=479647196
ヶ (pronounced か)
"An abbreviation for the kanji 箇 (ka), the counter for months, places or provisions.
Note that ヶ is a graphical abbreviation of the top of the character (⺮, 竹), and is pronounced カ ka (or ガ ga), not as ケ ke."
You'll mainly see it when a span of several months is specified, e.g. 一ヶ月（いっかげつ） - a duration/period of one month.