Upvote:1
In this case, you are going back in around one year. So, your intended length of stay is one year.
The arrival card is for a regular tourist, but people coming in on student visas may or may not need to fill it. If you are unsure, you can leave it blank and fill up if they ask.
In my personal experience, coming to Singapore, for work, I had to fill in the regular arrival card. Since I don't have a planned return date, I didn't fill that and it was perfectly Ok.