:set guicursor=i:blinkwait700-blinkon400-blinkoff250
This changes the blinking of the cursor in insert mode (use nvi for normal, visual, and insert mode). Adjust the time to your needs.
From the documentation help guicursor:
: blinkwait - , blinkon - , , blinkoff - , . msec. , . : "blinkwait700-blinkon400-blinkoff250".