For most clogs drain cleaner works on the clog within 15 minutes to an hour.
However for more stubborn clogs it may take the drain cleaner chemical overnight to fully clear the clog.
Drain Cleaner is safe to leave in the pipes overnight and will do no harm to the pipes.
I prefer Drano brand drain cleaner which works the best.
Drano clog clearing products including Drano Max Gel is safe for old pipes and new pipes.
The Drano Clog Removers are powerful chemicals but will not cause damage to metal or plastic pipes whether old or new.
All Drano Clog Removers are septic and sewer safe.
Drano Max Gel is safe to use in a septic system.
The Drano Max Gel is safe and compatible with your Septic Systems bacteria and will not harm the good bacteria in your septic system that breaks down the waste.
I have a septic tank and septic system and use Drano Max Gel to clear clogs and also other Drano Products and never has it caused an issue with my Septic Tank or Septic System.
All Drano products are septic safe and can safely be used in septic tanks.
Just make sure you have your septic tank pumped out every 5 to 10 years to prevent a sewage backup and use some Rid X in the septic tank to help break down the waste faster.
And your septic system should provide you with many years of use.