Generally, the sequence of undo-able actions is segmented by command. When
entering Insert mode, everything typed until exiting Insert mode is part of
a single undo-able segment. If you are going to be typing in Insert mode for
a while though, you may want to break it up a bit. Without leaving Insert
mode, hit ctrl-g u to mark a break in the sequence of undos.
For example, starting in Normal mode and then typing
iabc<CTRL-G>udef<CTRL-G>ughi<ESC> will leave the buffer with:
abcdefghi
Hitting u once will leave the buffer with:
abcdef
Hitting u again:
abc
Hitting ctrl-r:
abcdef
See :h i_CTRL-G_u for more details.