'I guess I could write an example if you want.' 'Can you guess how many pennies are in the jar?' 'At best guess, it will take me 3 days to complete my assignment.' These are 2 different ways of using 'guess'. The first example you can use informally in conversation, which is a way of agreeing tentatively. The second and third sentences the more regular use, conveying an approximation or estimate.

I guess you could use it whenever. I mean, my guess is that it is another word for "suppose" in some cases. That is unless you are asking someone to figure something out. Such as: "I bought a cake! Guess which flavor." When they answer, "Chocolate!" that is their guess. They have chosen "chocolate" to be their answer. Hope that is more helpful than confusing.