How long will it take for me to ride 100 km on a mountain bike on regular road with gentle slopes?

An average rider can probably do around 15km/hr on a mountain bike.

From the looks of your avatar, about 100 years.
in america that would be 60 miles, if you averaged 15 mph that would be four hours. If you did 30 that would be two hours.
It really depends on your ability. Truth is, if you aren't a very physically fit person, you won't get anywhere near 100 km before you are unable to ride further.
Depends on whether the slopes are more up or down.
Since 100 km is 62 miles, if you average 10 miles an hour you'll be on your way for about 6 hours. At 12 mph it'll be 5 hours and at 15 mph you can do it in 4
hey dumb asss what is your average speed?
4-5 hours

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