Just as clarification to the previous comprehensive replies, there is no '-5' speed displayed after a flap 5 takeoff, only '-1' and '-UP'. Retraction from 5 to 1 can be initiated above V2+15 (the "white bug") and accelerating.
Similar for a flap 15 departure: Retract from 15 to 5 when above V2+15 (and accelerating), 5 to 1 at '-5' and 1 to Up at '-1'.
A flap 1 takeoff is slightly different, retract from 1 to Up passing '-1'.
Reference: FCTM, 3.26.
Regards,
PA28pilot (who also flies 737s!)