Advice please - how much computer time is ok?

[hobnob]

My ds (just 4) likes nothing better than to copy words and short phrases onto the computer (his idea, not mine) and would happily do this for ages if I let him. NOW, the thing is, while this is clearly a Wholesome and Educational Activity, I sort of think that sitting in front of a computer might be in some way damaging to the health/eyes/brain/soul - but I have no scientific evidence for this. Am I just a ghastly old Luddite? Or are my instincts sound? How long would you think is suitable? I would appreciate some thoughts on this.

didn't you watch Child of Our Time last night - it was all about this and I don't really think he came a to a massively negative conclusion, other than if it is at the expense of physical play, then it is bad...... so in conjunction with enough running around outside, its probably OK in moderation.....

if he is learning skills on the pc rather than blowing up aliens on an x-box then i'd be happy, and let him do 30mins or so a day.

Thank you for your kind responses. Yes, blowing up aliens would be in another league of awfulness altogether! Sadly that was the one C of our T I've missed - must try and get hold of a copy.