I said shshshsh! I know I could say shush, but there are times when I really have to say shshshshsh. What would be the best way of writing this. Maybe Shhhhhh?
It's a transliteration of a sound, so write what you hear. If it's one long sound you write
(however many Hs amuse you)
If it's a short one, you write
with three Hs. One or two look like a typo (Sh! and Shh!). Four and it becomes the long version.
Sometimes I actually say the word "Shush!" so I'd write that word, but that's not the shhh! sound you're after.
If your speaker is actually repeating the short sound over and over (so there's absolutely no sustain on the hiss, as if you were trying to say "she" and stuttering), then your first example is correct.
He began to whimper.
"Shshshshsh, baby, don't cry," she soothed him.
Shush is a verb. The Oxford English Dictionary definies it as:
To call or reduce (someone) to silence by uttering the sounds denoted by sh-sh.
Sh is also a verb. The OED defines it as:
To reduce to silence or tranquillity with the sound of ‘sh!’, or attempt to do so.
Now what is interesting are the examples the OED gives. The first is from P. G. Wodehouse and uses the vers sh as we would expect:
She uttered a ‘Sh!’ of such significance that Grayce instinctively lowered her voice. ‘Mrs. Molloy!’ ‘Sh!’ Grayce might have retorted that she had sh-ed and that if she sh-ed any further she could become inaudible.
The second example is from Boyd Cable and contradicts Lauren's stuttering example:
Ainsley ‘sh-sh-sh-ed’ him to silence.
I understand this to mean that Ainsley is not stuttering but shushing "him" through a prolonged sh.
Consequently the OED lists this variant under "Forms: Also sh-sh-sh."
In Google the variants return the following number of hits:
shh - 5.210.000 shhh - 2.680.000 shhhh - 1.130.000 shhhhh - 672.000 (and Google asks if I mean "shhh")
Searches with variants of "sh-sh" (in quotation marks) turn up stuttering like "Sh Sh Sh Shark Attack!" or abbreviations like the career portal "SH SH", but no shushing, so usage appears to contradict the OED and confirm Lauren's answer.
Searches were performed from Germany on Google.de and will return different results for other user profiles.
As a result from my research, I would use "shhh" (with three "h" and no hyphen).
And +1 to Lauren, because I agree with her :-)
Adding to above answers: You could add description of the shhh, to specify its length, intensity, feeling, etc. and to help set the mood. Examples:
Scary: She laid a shaking finger to her dry lips. "Shhh," she said, as quietly as possible, like a ghost rustling the curtains.
Funny: "Shhhhhhh!" he hissed loudly, like the brakes on an old locomotive.
Sexy: "Shh," she purred, putting his finger to her moist lips.