I seem to remember a similar discussion in a thread about restaurant tipping in general, if you want to search out more opinions. (Not Applebees but maybe Outback or Carrabbas which have similar situations), for curbside service, I estimate a tip of about 10%+ and round the total up to the next dollar.

And for those who have not seen them yet, tip jars are becoming more and more prevalent at many fast food locations, even for those workers not being paid as tip workers (some companies specifically prohibit them, possibly because of payroll tax reporting issues). My impression is that this trend has been spreading from the west coast eastward. I saw them en masse over 6 years ago in California.