scart leads carry an analogue sytem, so theoretically you should be able to go a good bit more than 3m. If I were going to go for something very longI\'d make it up myself and use premium cable, as the drop in signal strength over a long run might prove problematic
25 metres should be ok with off the shelf cables.
http://www.nexxia.co.uk/Hi-Fi&TVaccessories/scart_plugs_&_cables.htm
the theoretical maximum recomended run for analogue cables is around 100metres, but only with the very best cable and connectors, when selecting cable the thing to look for is transmission loss figures, these should be as low as possible, anything over 0.2db per metre and you will notice a major drop in quality over a long run.
standard a/v leads are to all intents and purpose the same as scart - just analogue, scart seperates the video compents tho.
what you doing with it?? (being nosey!)
you can get wireless video repeaters with a range of 100 metres or so, which will allow you to make a wirelss connection but they an be iffy depending on whats in between the 2 devices (ie thick walls!)
you around later this week marc??