Deciding your possession end-point is a private matter between you and God. My answers for how much to keep will likely be different from yours. Likewise, my reasons for casting away may be different from yours. My intention here is to share some of my reasoning in hopes that it may help you in your own journey.
After finally understanding on a heart level that there is a one-to-one relationship between the possession of things and the consumption of time, I determined to get serious. My complaining about a lack of time did not fit with the family rule about not complaining about anything you are not doing something about. Therefore, I had to put up or shut up. Too frustrated to shut up, I was forced to eradicate stuff until I did not feel the need to complain anymore.
The first thing I did was to make a list of everything I needed. (A quick, easy method of life reorganization could have been to consider anything that did not make the original list to be automatically out.) After making this list of "needs" (that was remarkably long), I began crossing out items as I was convicted of the non-necessity of each. This list of necessary items will be different for different folks.
In case going through every item we owned with this list in hand was not quite enough motivation, we have taught ourselves to ask a question that has been even more beneficial. 'Is the convenience or pleasure this item brings WORTH what it takes to maintain, clean, house and shuffle this item around until I use it?'
A negative example would be the three-tiered plate stand that I use about twice a year for special girl-birthdays. Truth is, the thing is a hassle to schlep around and to have taking up so much space in my pantry all year just for those four hours a year that it is used. Yes, it is pretty, yes, it is the perfect thing for serving the petits-fours and chocolate-dipped strawberries. But it isn't worth all of the rest. I am finding that most things are not.
A positive example is the jogging stroller. With the extra large tires. That does not fold up. It is big and bulky, yet I have decided to keep it because it gets used almost daily. Walking the quarter mile driveway with a fifty pound three-year-old makes it worth it's space in gold to me. It has enough value to me to justify it's continued presence.
Another question that has proved helpful is, "how many of these can I use at one time?" or more importantly, "how many of these do I use at one time?". I have six laundry baskets that are often being used for some good reason all at the same time. However, when I realized that my five whisks and five cutting boards in various sizes had never been used all at once, most of them were slated for dismissal. I saved only the heavy-duty spoon, and two boards, one for meat and one for vegetables and bread. While making last night's dinner, I had to wash the vegetable cutting board six separate times in the process, but I only had to store it once.
A helpful activity may be to sit down sometime when you are feeling refreshed, and as objectively and unemotionally as possible, think and pray through what the callings are in your life during this season. The 'things' of our lives are simply tools to further God's work in our lives, and to help us do whatever it is He desires for us. Those things, when not prayerfully added or removed can be a disastrous hindrance to our ability to walk in obedience. Since they can also be tools He uses to help us minister to others, we need discernment from the Holy Spirit, and a heart of obedient trust to know which items are which.
Based on lifestyle differences (city/country, large family/small family, work & school at home/work & school away), and seasons of life ( ages of children, tight finances, moving often) we will each be led in different areas to keep or discard the chattel of our lives. The first key is to seek Father's face and desire to know His heart for you and your family, trusting that He will lead you. The second key is to obey what He leads you to do. It is one thing to be convicted about the level of extraneous baggage our family is carrying with us through life, and working to change it. It is quite another to know we have too much and to ignore the promptings to change it.
Here's to traveling light for His glory!