Most will be three months initially with an extension where required. Obviously there's shorter ones available too, but 3 months seems to be the most commonly quoted initial length.

Yeah. Three months is usually the minimum for a contract. 6 months is more common in my experience. And then there is the chance of renewals on top.

Usually 3 to 6 months (unless you specifically go for a horrible couple week contract) and every contract I've worked has rolled on or been extended.

Have you had any issues with IR35 with regard to long-term contracts?

Never, ever, ever nor has any contractor I have ever worked with. Saying that the longest contract I've worked was 1 1/2 years.

IMHO the IR35 stuff you see is just companies trying to sell you advice and consultation and build a market around it.