What is the length of the longest land worm ?

The Australian earthworm.

'The longest worms on land are some of the earthworms in Australia. These earthworms, belonging to a group called the Megascolecids, are characteristic of the southern hemisphere; they get to be as much as 2-3 m long. At this point most of them are threatened by the modification of the local landscape where they live.'

It gets better (or worse, depending on one's viewpoint, I guess).

"The longest of such species was found in 1930, which measured 7' 2" in length, and over 13' when naturally extended. The eggs of this worm measure 2" to 3" in length, and 3/4" in diameter. In November, 1967, a specimen of the African giant earthworm Microchaetus rappi measuring 11' in length and 21' when naturally extended was found on the road between Alice and King William's Town, South Africa. The average length of this species, however, is 3' 6" in length, and 6' to 7' when naturally extended"
Source: Guinness Book of World Records