Let your story needs and reader expectation drive your timing. Since your book is about time travel, readers will crave it. They won’t bat an eye if you take a chapter or two to build your story, but after that they’ll probably start wondering, “Hey, where’s the time travel stuff I bought this book for?” If your pre-time travel portion exists mostly to establish the current world so we can understand the psychological impact of leaving it, be quick about that task. However, if you’ve got substantial action or story in the pre-time travel part, readers will be jazzed enough about the here and now to wait as many as four or five chapters before wondering where their time travel is. In the 382-page bestseller Miss Peregrine’s Home for Peculiar Children, the protagonist doesn’t go through the time/world gateway until page 124, mid-Chapter 5. Many readers don’t know another world is behind the mystery of monsters in the first 123 pages, so that timing works for that story and its audience.