depends on what 'gently' means to you. and how old/used you want to go. 2 yrs old? more? most used bikes for sale that are less than 5yrs old have under 5,000 miles. so that's what i'd pinpoint the norm at: less than 5yrs, less than 5K miles. so in that group, if you want one more 'gently' used than others, you may look at bikes that just have around 2K miles or so. of course there's a lot of other things to consider that would make an older bike with more miles more attractive than a newer bike with fewer miles, but this is just a general guideline that i think you're asking for. hope that points you in the right direction.