Tricky. The first rule is "if in doubt, don't water"! Unless your Lithops is so horribly shrivelled that you fear for its death then it has no immediate need for water, but watering a Lithops that doesn't need water can mean instant death.

Lithops in strong sun will often develop wrinkles on their sides during the day. Often these are gone by next morning and there is no need to water. If the sides are still wrinkled the next morning then you may want to give some water but there is no harm in waiting a while longer.

Often you cannot see wrinkles on the sides because there are some remains of old leaves or the plants are mostly in the soil. Then they can "sink" into the soil without you noticing that they are thirsty. This is caused because the sides have wrinkled quite a lot and the whole plant has shrunk. When your plants try to shrink below the soil you can water them. Watch out for small seedlings which will be pulled down into the soil in strong sun, they may just be hiding from the sun and not really thirsty, but it is generally safe to water young seedlings frequently. In hot dry climates some growers spray a little water on their Lithops quite frequently. I don't do this in my climate, I'm not sure whether this is helpful in your climate or not.

The last rule is that once you water, you don't need to water again. If the plant does not respond within a day or two then it has a problem which will not be helped by more water. Maybe it is short of roots for some reason such as being repotted recently. You may need to shade it for a week or two to let it settle down, or check to see what other problem there might be such as pests.

Flowering does involve some water loss for the plant. Flowering is also an indication that the plant is in its main growing season, so it is generally safe to water unless the plant is already bloated. In cooler climates, the light is poor and the weather is cool especially for the plants which flower later, so watering can cause problems. In these conditions it is usually best not to give any more water once the buds develop. Lithops will only abort a bud at this stage because of a major problem.