这里为什么选 what

what在从句中做主语，取“所……的”之意。当然翻译的时候可以根据意译：看起来是好长一段时间之后，那个人苏醒了。

因为 后面的he was making progress.和 he had got used to the life there共同作为told he had got used to the life there 的宾语
A因为通过分析句子结构可知道after _______ seemed a very long time中缺少引导seemed a very long time的主语和after的宾语，而when只能是引导时间状从的

what seemed a very long time做介词after的宾语，是宾语从句，what在从句中做主语。
如：
1.After ___ seemed ages,the newsman disclosed the facts.
A.there B. which C.what D.that
2.In front of ___remained of my old house, I took some pictures and told my son some funny stories in my childhood.
A. which B. the place C. what D. where