It doesn't have to be a long time, 10-15 mins a day is enough to make a real difference. It is often more effective to study a little bit everyday than to spend a long time studying only once a week.
Ambush studying is where you make learning part of your everyday life. For example, make yourself some bookmarks with the phrase "Tha mi a' leughadh." (I am reading) on them. Put them in the books you are reading, Label the food in your cupboards with their Gaelic names. Label the drawers in your dresser with which type of clothing is them. Put sticky notes with phrases around your house. "Nigh do làmhan" (wash your hands) next to the soap, "Oidhche mhath" (good night) on your bedside lamp. In this way you see and use Gaelic all the time without having to make special time to study.