The S&P index has been in a beautiful trend for quite sometime now and, as with any trend, new entrants may become increasingly more anxious of getting in too late.
From the very simple analysis here, it looks like the index has some way to go before one needs to prudent and contemplate exiting the positions.
Here we have weekly chart that is making all time highs. The fib extension from last peak to trough suggest 2100 as the next major level and the rally doesn't look too overbought from . The vol is low (The reasons for low vols are well documented and that's a topic for another discussion).
As such, it is to early to be worried about booking profits or even contemplating whether it's too late to enter.
The music is still playing