(1) A present tense verb form is used in the by-the-time phrase when the main clause expresses present or future perfect timing. (2) A past verb form is used in the by-the-time phrase when the main clause expresses past perfect timing. They have always eaten dinner by the time we arrive.

Can you start a sentence with by the time?

Now, let's go to your question. Starting a sentence with by is fine. The subordinate phrase is a prepositional one which acts as an adverb to modify the verb consume.

What is the present time expression?

We use the present continuous with time expressions such as: now, at the moment, at present, today, tonight, this morning/afternoon/evening, this week/month/year & while. These time expressions have the following basic positions: At the very end of a sentence, that is in the complement.