I always politely suggest at least one definite possibility when scheduling is required. I append it with "I am also free between ---" so that other options are available.
The reason I do it this way is because if you say "whenever," especially over email, you can get into a trap of back-and-forth where you gradually narrow down scheduling over 3 or 4 additional emails - adding friction and cognitive load to all your communications. You should be trying to move the ball forward as fast as is safely possible, and giving a definite time right off the bat usually reduces the back-and-forth to "That works. See you then."
The reason I do it this way is because if you say "whenever," especially over email, you can get into a trap of back-and-forth where you gradually narrow down scheduling over 3 or 4 additional emails - adding friction and cognitive load to all your communications. You should be trying to move the ball forward as fast as is safely possible, and giving a definite time right off the bat usually reduces the back-and-forth to "That works. See you then."