Show all results for past N "insert batches"
if i understand you correctly, here will do:

select user, sum(RT) 
from test2 a
inner join ( select distinct(time) from test2 order by time desc limit 2) t
on a.time=t.time
group by user


The 'N' for number of latest batch is determined by the limit 2 in the subquery t.

For example, if i run the above query, showing data for the last 2 batches, the output is:

Dailymotion 8.11
Facebook    1.44
Google+     0.98
Instagram   0.15
Metacafe    0.80
Twitter     0.75
Youtube     0.48


But the query above won't show any users who don't have data in the last 'N' batch. If you need to show those users as well (as '0' RT), you will have to join the above query with your users table.

P/S: The test2 table in the query above contains the exact data as in your first table in your post.