How much energy can a RF harvester capture from a mobile phone
Zero unless the phone is transmitting, which it isn't most of the time.  Note that the circuit in the link you posted only works 1. shortly after the phone is turned on, 2. while talking on a call, 3. while sending a text.  So for any given 5 second period, your harvester is almost certainly not going to get any energy from the phone at all.  Now, when your phone is transmitting data to a cell tower (perhaps you are using it to talk to someone) then the transmit power will depend on the distance to the nearest cell tower.  The transmitters in phones are actually rather powerful, I think some are even capable of transmitting at a watt, though the battery drain when transmitting at that high of a power level is significant.  My guess is that if you get lucky and the phone is transmitting at high power, you could extract a few tens of mW.  More likely you would only get a few uW, especially if the phone is connected to a local wi-fi network instead of to the cell network.  However, most of the time you're going to get nothing.