With today's poll from BMG, it looks like Labour's poll bounce under Burnham has been on average ~4.7 points. A 2 point bounce when Starmer resigned and a further 2.7 points when Burnham became PM
Labour's gains seem to come at the expense of the Greens, who are down two points since Starmer resigned:
Labour 24.7 (+4.7)
Reform 24.6 (-1.5)
Conservative 20.0 (+0.9)
Green 11.0 (-2.0)
Lib Dems 10.6 (-1.3)