The next day!
Whenever you write something, make sure you do your read-through the following day. I always try to write in the mornings (because I'm fresher and more alert), then leave it alone for the afternoon, possibly a look over in the evening with basic edits - and finally, I always try to do my proofreading in the morning.
Why? Because when you read something you've just written, it's almost impossible to spot the mistakes; your brain will read it as you just thought it. You won't be able to tell if it flows correctly - maybe it doesn't even make sense at all.
But, the next day you will have forgotten how it went; the grammar, the words, the flow, the paragraphs etc. will all be new to you again and, if you don't like it, you have the opportunity to fix it and read it again the next day. Clients don't always understand this process, but it's more important to get it right first time than for them to think your cat wrote the copy!
There are times when I've needed to get something off to a client quickly (after reading several times), and they've sent it back because they've found a spelling mistake! That's not the best way to showcase your writing and can be quite embarrassing.
A spell check tip I learnt from the great copywriter, Bob Bly, is to read the entire piece backwards. Because the writing won't make any sense, you tend to look at each word individually; this way you're far less likely to scan over the mistakes.
Of course, if you have someone on hand to take a look through, don't be afraid to ask!
Now where's my wife?