Use terms such as beginner, intermediate, or advanced user. Then, give a few examples of things you commonly utilize in Excel for such as creating charts/graphs, making pivot tables, or analyzing data. Your response will help the interviewer understand what type of training you should be provided, should they hire you.
"I have used Excel for about four years now, and would rate myself as an intermediate user. I am capable of making charts and graphs, and some other intermediate level tasks."
"I am newer to Excel in a professional setting but have used it in a personal setting for quite some time. I am going to assume that I am a beginner level user in Excel."
"I have approximately twelve years experience with Excel and would rate myself as an advanced user. I can create pivot tables, charts/graphs, and analyzing data. Are there specific tasks you are looking for me to complete in Excel?"