Repair times:

4sec per 1% + 0.2 * level

SMG/AR/RF : 8sec * 1% + 0.4 * level

cost before critical :

Manpower = ROUNDUP( a * (lostHP / totalHP) * dummylink

Parts = ROUNDUP( b * (lostHP / totalHP) * dummylink

Cost when critical :

Manpower = ROUNDUP( a * (lostHP / totalHP) * {1 + (0.30 - currentHP / totalHP) * 4} * dummylink

Parts = ROUNDUP( b * (lostHP / totalHP) * {1 + (0.30 - currentHP / totalHP) * 4} * dummylink

where

a= AR:40 / SMG:45 / RF:35 / SG:65 / MG:75 / HG:20

b= AR:14 / SMG:12 / RF:16 / SG:32 / MG:30 / HG:7

In short:

Basically, every dummy link lost increases repairs significantly and repairs aren't linear. Which is why shot out SMGs can take hours upon hours to repair, while scratch damage is done in seconds. Basically repair when you can. Data taken from KR wiki. I dunno why I put this here but it may be useful to someone.