If you believe in the potential that will soon become a reality with the Bitshares DACs, then invest whatever you can afford. Don't stretch your personal finances to the point where you commit money you need for paying bills, etc., but do think about how much represents a significant stake for you. Remember that if you own twice as much, the value will only need to climb half as high for you to become a millionaire or billionaire.
There is a chance you might lose all of it. I happen to think the chance of losing it is very low, given the outstanding team and progress I have seen thus far. But do the research and make that decision for yourself.
There should be a whole suite of DACs coming out in the next few months. Remember that the AGS donation period will be ending in a few weeks and PTS will soon be cut off and reformed into a new product. There will never be more than 2 million (quantity) of either one in existence and probably significantly fewer PTS by the time it is moved to DPOS, so if these products are even halfway successful, your PTS should be a in great demand and your AGS should become a profitable holding.
Some peoples' patience has been tested as these products are being developed, so the price of PTS and value of AGS are at near-historic lows. You couldn't pick a better time to get in than right now.