That's a large range. 1400-1900 and 1-6 months.
1 month, probably less than a week if you solved for and hour or two a day.
6 months maybe as much a 1 month if you solved an hour a day, 5 days a week.
I have no idea though, it would likely depend on the player themselves as well as age and experience. A kid rated 1900 who quit for 6 months would have a lot more rust than an adult who's been playing for 20 years rated 1900 who quit for 6 months.
How long (days or # of tactics) do you think it takes to regain tactics form for about 1400 to 2000 USCF level players? Obviously this depends on many factors but assume the tactics are of medium difficulty and that the player has not played chess for about 1 to 6 months.