"It took twelve months."
【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.