I'm curious about the usage of '에' after '쯤' and the difference between '여름 때' and '여름 동안'.

Hello! I have two questions about Korean grammar. The first question is about using '에' after '쯤'. For example, I understand that it can be used like '난 1월쯤(에) 한국에 갈 거야'. However, it seems that '에' is not used when '쯤' replaces '동안'. For instance, in '저는 4시간쯤에 공부 했어'. I'm curious if this usage is correct. The second question is about the difference between '여름 때' and '여름 동안'. I would also like to know the difference between '방학 때' and '방학 동안' in a similar context. I got confused about these points while talking with foreign friends in Itaewon. If anyone knows the answers to these two questions, I would appreciate your advice!