How much slack to leave in chain?
 

I tightened my chain today but was not sure how much slack to leave.

I know it should not be tight, so i tightened it all the way then let it out until I could visually see some slack.

I am not sure if that is enough though.

The chain should have 1 to 2 inches slack (loose movement)(disclaimer: from memory, the precise amount is somewhere in the manual though). Mostly, slack is not constant - when you turn the rear wheel, at one point you will have almost no slack, and somewhere else you can have 1.5 inches slack.
Better a little too loose than too tight.

Can you get the rear wheel off the ground? Will assume you can. What I would do is the following.
Find the mid point between the sprockets and measure the bottom of the lower length of chain from the floor. Rotate the tire a bit and repeat. Repeat again and again till you have found the tightest(least slack) position of the chain. Measure that..........now lift the bottom length of the chain and measure the difference. I would set the slack at 1 inch and call it a day. Hope this helps. Lynda's GZ ran nicely with this measurement.

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The owner's manual says 5-15 mm, and there's 25.4 mm to the inch, so it's around three fifths of an inch max. You'll need a metric tape to get it accurate.(BTW, that measurement is from chain pulled down to chain pushed up)

Chain slack should be 9/16" (15mm) up & down movement
at the center of the chain.Rotate rear wheel to get to the slackest point.To loose is dangerous as the chain can come off with disastrous results. :)

How often should the chain be adjusted in normal riding conditions?

It should be checked/cleaned or adjusted every 1000 km (600 miles). ;) :)

Gotta disagree with you Blaine. The slackest point may not allow for any slack as the chain may be too tight as it rotates around the sprockets after the chain adjustment.
A chain that is too tight will interfere with rear suspension movement and may also do undue wear to the chain, sprockets and the shaft the front sprocket is mounted on.

adjust the chain at its tightest point
to tight may stretch or brake the chain
to loose will increase the wear on the sprockets

Oops......Meant the tightest point NOT the slackest.....Good catch W.W. :)