Yes, around five half-lives for most of the Test C to clear, but blood levels can drop below your baseline way before that.
By week two roughly 70% of the original Test C will be gone...yes some remains, but what gets released daily is much lower.
As to injecting 600 to750 of Test C before leaving with the assumption is that a larger dose extends coverage.
Technically, a larger dose can keep Test levels elevated longer, but it doesn't change the ester's half life. You're starting at a higher level and following the same declining curve. Possibly extending coverage but... also increasing the risk of side effects from the higher initial Test and Est levels
By weeks two and three, Test levels will have dropped. Some Test C remains, but that doesn't mean blood levels are adequate.
Basically risking elevated Estrogen and it's side affects (like low libido

) early in the trip, followed by a Test crash toward the end.
My opinion..stopping Test C eleven days before departure allows roughly 1.5 half-lives to pass, lowering Test levels before Test U which has a longer release curve with initial peak around seven days (give or take)
Work with overlapping release curves rather than over dosing Test C hoping it lasts the entire trip
Or...just get a script...maybe?