В то время как Анна сидела в ночной тиши, она думала, сколько времени Джордж, должно быть, провел здесь один.
As Anna sat in the still of the night, she thought about how much time George must have spent here alone.
GC: The verb SPEND needs a noun object, so in your sentence HOW LONG cannot be the object because it is an adverb phrase. In other words, you can't ask: "How long did he spend here?" but you can ask: "How much time did he spend here?"
Also, the use of WHILE with the progressive puts too much emphasis on the simultaneity of the situation. It would sound better in a situation like "While she was sitting at the table feeding her baby, she was monitoring the tv in the living room for news of the terrorist attach." Here the progressive gives the reader a of two active actions going on at the same time. However SITTING and THINKING are both rather motionless, so it's better to use AS with the past tense.
@Uly Marrero, не знала таких деталей, спасибо большое! (-:
всегда пожалуйста)