You can use the phrase "I don't want to take up too much of your time" at the beginning of a conversation, to show that the conversation is going to be short:

Hey, Melanie, I don't want to take up too much of your time, but I need to go over these sales projections with you. Do you have a second?

You can also use this phrase to end a conversation politely, in the same way as the similar phrase "I don't want to take up any more of your time":

Listen, I don't want to take up too much of your time, but it was great talking to you.