That's an interesting question! If I understand correctly, those symbols would be read as the last syllable being extended, to emphasize the importance of the word?
I think what's most often used is a bunch of exclamation points - "so happy!!!!"
I've also seen people repeat the last letter, as in Sooooooo delicious!
I don't think there's any rule about it, though. If you visit some English-language internet forums, you will see many creative ideas. ^_^