There is no definitive answer to this question as it depends on a number of factors, such as the type of bike you are riding, the terrain you are riding on and your personal preferences. However, there are some general guidelines that can be followed in order to ensure that your chain has the correct amount of slack.
First, it is important to note that there should always be some slack in your chain. This is necessary in order to allow the chain to move freely as you pedal and avoid any potential damage. If there is too much slack, however, it can cause the chain to fall off the sprockets or become tangled. Too little slack can also lead to problems, such as premature wear or damage to the links and bearings.
To find out how much slack is ideal for your bike, start by putting the bike in its highest gear and pedaling forward. With one hand, hold the bottom of the chain close to where it meets the rear sprocket. Use your other hand to slowly lift up on the chain until it starting moving freely on the sprockets. The point at which the chain starts moving is where you want your ideal amount of slack to be.
Remember, these are only general guidelines and you may need to adjust according to your specific bike and riding conditions. If in doubt, consult a professional mechanic who can help you determine exactly how much slack is right for your situation.