expression
for months; many months
Emphasizes a long duration of multiple months, often with a sense of 'for months on end' or 'for many months'. Used adverbially.
彼は何ヶ月も入院していた。
He was hospitalized for many months.
この問題は何ヶ月も解決していない。
This problem hasn't been resolved for months.
何ヶ月 means 'how many months' as a question, while 何ヶ月も means 'for many months' as a statement.
Combination of 何 (how many) + counter ヶ月 (months) + particle も (as many as). The ヶ is a simplified form of the kanji 箇, used as a generic counter.