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Professional studio announcement timeline:
@E3: We're releasing in time for Christmas. @Christmas: We're releasing this Spring. @Spring: We're releasing this Summer.
@E3: We're releasing in time for Christmas...
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How to schedule a game:

Also, before the jam starts, make a schedule listing EVERYTHING you need for a game: 
Gameplay, Art, Sound Effects, Music, UI, Tutorials, Menus, Level Design, etc. 

Now figure out how many hours you have to make your game. (In this jam, 168hrs.)
From that number, subtract all the hours you need to live (sleeping/work/eating/bathroom/etc...) [See next tip for help here.]
Take that remaining number and cut it in half.

Divide the time you have left among the items I listed above. (Give more time to unfamiliar tasks and remember that "searching for pre-existing art/sounds/music" can take as long as making it yourself.)

This will give you a rough estimate for how long you have to make your project.

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How to improve: 

Here's a trick I've had my students try: Tonight, make a list of EVERYTHING you will spend time on the next day (including sleeping, checking email, work/homework, social media/internet browsing, eating, bathroom breaks, gaming, chatting with friends, chores, showering, brushing your teeth, etc.) For each of these activities, write down a guess of how much time you will spend doing it.

The next day, as you are about to do something (like go to the bathroom) write down the time you are starting. When you are finished (or switch tasks) write down the time you stop. (For items that you do multiple times, list each start-stop pairing.) When the day is over, add up all the time you spent on each item and compare it to how long it ACTUALLY took to how long you THOUGHT it would take. This gives you a basis for estimating how far off your time management guesses are. Chances are, you'll be close on some routine items (like brushing your teeth) and WAY off for items that are unusual, highly variable, or new to you.

Now judge your personal experience making games. Is this closer to being "routine" or is it a "new" skill for you? That should help you figure out how far off your time judgements are for making games.

If game dev is a newer thing for you, I recommend (for this jam) aiming to make a game you can COMPLETE in 1 day-2 days. That gives you more room for error.