If Makar walks in 27/28 that'll be the end for this core and the time to start building up futures. If he stays, they'll keep trying for a couple more seasons with MacK/Nuke/Necas/Makar/Toews. At latest the rebuild will start in 30/31 imo but most likely before then.
24-25 best chance for a cup
25-26 still a good chance
26-27 a good chance if Necas stays, otherwise a low chance
27-28 rebuild if Makar leaves, otherwise a low chance
28-29 a low chance or miss playoffs
29-30 a low chance or miss playoffs
30-31 rebuild