Several months; a period spanning multiple months (exact number depends on context).
数ヶ月前から続いている仕事がようやく終わった。
The work that had been going on for several months has finally finished.
このプロジェクトは数ヶ月にわたって実施される予定だ。
This project is scheduled to run for several months.
彼とは数ヶ月会っていない。
I haven't seen him for several months.