With Sustained Fire, your firing rate increases by +15%/second, capped at 100% - so you need just over 6 seconds of staying still to reach max. firing rate.
With Improved Sustained Fire (introduced in 1.15), firing rate increases by +25%/second, capped at 150% - will reach the cap in 6 seconds.
See Gun Speed (shots/second) and Speed Perks for how it stacks with other Speed perks
