The whole journey (will have lasted) a full week by the time it ends in France.
by the time 到...时间为止,要用完成时态,
又因为从句里ends用的是一般现在时,所以主句用将来时,
结合起来,就是,将来完成时态,
翻译:当它(旅行)在法国结束的那个时候,整个旅行将已经持续一整周了.