You need to inform tenant 90 days (for selling house), tenant might give you 28 days notice to quit once they found a house, so is difficult to say when they will move out, the time frame is 90-28 days.
IF you want your property to expose to maximum exposure and marketing, best to market without tenant and the agent can bring buyers any time to see the house and do building inspections, etc.
For fixed term, no longer 21 days, I think is more than that like you need to inform them 90 days in advance if you are not going to renew the fixed term (need to check) with valid reasons.