I'm not saying it has to be around 7 years, I'm just saying it's generally not a good idea to start investing all your money into the stock market when it's at or near all time highs.
There's even an urban dictionary word for it. "BTFATH" - Buy the fucking all time high. http://www.urbandictionary.com/define.php?term=BTFATH
Or to put it more concretely, there is a long chain of "all-time highs" before the last all-time high. You would miss out on all that investment time if you avoided the market based on all-time highs. And then a correction comes -- but is it the one you were expecting or is there a bigger correction in the wings?
In the end, one never knows. None of the rules of thumb are reliable. And you can shoot yourself in the foot far more by trying to time, than by just buying and holding.
I hold my suggestion to the OP, keep a portion in cash.
It's been shown that if you have a lump sum of money, it's most often better to dump it all in the market as soon as possible, rather than gradually buying in.
Of course you shouldn't put 100% of your money in the stock market, your advice was reasonable.
The pitfall with "Thinking the market generally goes up and one should invest in it is not timing" is, the thought only occurs after the market has gone way up, rather than occurring unbiasedly between instances when the market has gone way down or way up.