This is a great question, and you should know that a cheap garden sprayer will not have this information in its manual. You should be very careful how much pressure you use because in case it is too much, you might damage your garden sprayer.
The best way to know how much pressure you should apply is by listening to your sprayer. Pump it up a few times and see how much liquid comes out of the sprayer. If it is not enough, try and apply more pressure in order to find out the best amount.
Do not overdo it because you will damage the rubber parts of the sprayer, and you will have to replace them. If these parts are not replaceable, you might even have to buy a new garden sprayer, and that is not a great option, especially if you have purchased a proficient one recently.