In general – and I cannot stress this enough: you asked for general, so I'm giving you general – multisyllabic names are often shortened to the first syllable(s). Hence: Jon (Jonathon), Rob (Robert), Will (William), Mike (Michael), Dave (David), Tom (Thomas), Doug (Douglas), Chris (Christoper), Alex (Alexander), Sue (Susan), Chris (Christine), Meg (Meghan), Nance (Nancy).
Sometimes, a name takes on a more informal tone by adding a -y suffix – although this transfiguration can serve to shorten a longer name, lengthen a shorter name, or keep a name the same "length": Cindy (Cynthia), Becky (Rebecca), Johnny (John), Tommy (Tom), Polly (Pauline), Candy (Candace).
Some common names have some rather odd exceptions, many of which have already been pointed out. I'll list Bob (Robert), Bill (William), Jim (James), Maggie (Margeret) and Jack (John). The last one maybe shouldn't even count toward your question, because it doesn't shorten the name.
Elizabeths can go by Liz, Beth, or Betty.
There's no set pattern, though, and it's generally best to go with someone's given name until they tell you otherwise.