How do I know when I should start buying stocks and bonds again?

You'll get as many opinions on that question as there are flavors of ice cream. So you locked-in losses in 2008, went to cash, and missed the run-up out of the last downturn. It's been quite a run since then, artificially goosed by central bank monetary policy on steroids...but don't let yourself now feel pressured to jump back in with both feet.
Nobody has a crystal ball, of course, and nobody can precisely and consistently time the markets. Most of the financial industry will tell you to go ahead now and fully allocate your portfolio based on your time frame (when you'll need the $), tolerance for fluctuation (which they call "risk"), and so on. And those are all valid factors but they completely ignore the extreme overvaluations we have today. 
I'm among a minority of advisers who believe this may well be one of the WORST times for those who've been out of the market to now jump in with both feet. Just look at the valuation ratios of today compared with 2007, 1999-2000, and just before the GreatDepression. That doesn't mean the markets are immediately going to tank tomorrow (they could). But it is does mean that we're likely going to see terrible average annual returns in stocks and bonds over the NEXT 7 to 10 years. 
So why not wait for a healthy pullback and then buy some assets at better prices? Or if you absolutely feel you've got to get in, then maybe "average" your way in with multiple purchases over a period of time instead of all at once.
You have realized losses which you locked-in by selling after the 2008 downturn. You also have "lost opportunity cost" from being in cash since then. But lost opportunities are much easier to stomach than a new set of catastrophic losses. And while the markets can certainly march higher from here, jumping in now at these elevated levels may just be setting you up for another round of what you went through in 2007-2008.
Hope that helps. All the best in your decision!

SecondHalf Planning & Investment

You may know this all ready, but there are two approaches to market decision making: fundamental and technical. The fundamentalists look at the economy ( I don't feel comfortable with it, do you?); the technicians look at price activity and market trends to make their decisions. I bring a bias to this discussion as a history major in college. While past performance is no predictor....., markets, I think do trade in patterns and can tell us a lot of things if we ignore CNBC, Fox Business and the other talking heads and press sensationalists. Based on supply and demand for stocks (equities), I am a cautious buyer currently of US and International equities for client accounts. Fixed income? No. Currencies and commodities are interesting to me.
Much of my strategy with clients and portfolios and my personal thoughts about the markets comes from the work of Dorsey Wright and Associates. Their disciplines are based on information in a book called Point and Figure Charting by Tom Dorsey. The book is not widely read by my peers: maybe that's why the herd mentality hasn't spoiled it's results yet. Some of their strategies are also available through ETF's that they co-manage. This reads like a commercial. I've used the discipline for fifteen years and it still seems to work.
You could also consider some of the available inverse market ETF's available, BUT stay away from the leveraged ETF's. There has been some interesting chart activity in the last couple of weeks among the rising interest rate/defensive fixed income ETF's and ht short term fixed income ETF's. If they fit your investment, not trading, criteria, theymay offer some interesting prospects.
My two cents worth.