One day ticket is not so popular in Tokyo and introduction was quite late if you compare to other countries in Europe. Then, none of the railway company has fixed one flat fare per ride, therefore sometimes it is not worth to buy an one day ticket. May be it is cheaper to use the SUICA or PASMO each time. Also, there is no universal one day ticket which you can use in Tokyo for all railway companies, except Tokyo Metro, Toei Subway and JR East which have their common ticket in Tokyo metropolitan area including Toei Bus and Toei Arakawa tram line.
Then, there is no ticket which is valid for 24 hours. If you buy at 16:00, it is valid till midnight.
Also please be aware about the validity of the ticket itself. If you buy the one day ticket from the ticket vending machine, it is valid only on the day of the purchase, which means you can not buy it in advance. Therefore if you stay in Tokyo for two days and you want to use the one day ticket, you have to buy each day or you have to go to the designated ticket offices which sell commuter tickets.
One day ticket for Tokyo Metro is JPY 600 and common one day ticket for Tokyo Metro and Toei subway is JPY 900. Tokyo combination ticket which you can have unlimited ride of Tokyo Metro, Toei subway, Toei bus, Toei Arakawa tram line and JR lines(Tokyo metropolitan area = 23 special wards) is JPY 1590. One day ticket just for JR East in Tokyo metropolitan area is JPY 750. Therefore it is slightly cheaper to buy Tokyo combination ticket than buying common one day subway ticket and one day JR East ticket, but if you are sure not taking Toei subway, better to buy separate tickets.