By default, PPLNS does not penalize/discriminate users based on their consistency. Every share has the same expected payout as every other share.
https://en.bitcoin.it/wiki/Comparison_of_mining_pools
PPLNS - Pay Per Last N Shares. Similar to proportional, but instead of looking at the number of shares in the round, instead looks at the last N shares, regardless of round boundaries.
PPLNSG - Pay Per Last N Groups (or shifts). Similar to PPLNS, but shares are grouped into "shifts" which are paid as a whole.
Thanks, but from what I read, casual users or users who suffer from downtimes should mine on PPS instead. I just wanted to know how much would be considered a noteworthy downtime for this matter.
What you've read is people who don't understand the payment method. The payout on your shares for PPLNS are not affected by *anything* related to uptime. If you mine for 3 hours, your expected payout for those 3 hours is the same whether you were mining for the previous 3 hours or the next 3 months. The expected payout for that 3 hours of mining is completely independent of any other factors.