In Momentum Deferred (S2E4), we see a particular shapeshifter becoming very weak and sick. Another shapeshifter comments that this is because he has been using one body for too long. Since this is only a few episodes after we initially see the sick shapeshifter take his current form (S2E1), we can pretty well presume that his time in this form has only been a few weeks of in-universe time. At the most, we might expect a few months.
In the same episode where he begins to show sickness, we also see that he can partially alleviate his symptoms by consuming raw mercury. However, this appears to be little more effective than applying a band-aid to a direct-hit gunshot wound. The effects of the sickness are less pronounced, but still rather noticeably debilitating.
Still in this same episode, we're introduced to another shapeshifter who continues to be a recurring character throughout season 2, and into season 3. However, this character does not change bodies during this time. He also does not exhibit the same sickness suffered by the other shapeshifter, which was attributed to having remained in one body for so long.
In Do Shapeshifters Dream of Electric Sheep? (S3E4) we are introduced to yet another shapeshifter. Through dialogue, we find out that this shapeshifter has maintained the same body for at least five years. It's even possibly implied that the shapeshifter mentioned earlier - the one introduced in (S2E4) - has been in his body for at least the same duration. Again, the latest shapeshifter does not appear to be suffering any ill effects from his prolonged period in this one body.
In fact, it is a critical part of the latest shapeshifter's character story that he intends to remain in his current form indefinitely.
Is there anything in standard canon, or official commentary or documentation, to state how long a shapeshifter can effortlessly maintain one form? Or, is there anything mentioning how shapeshifters can safely and reliably extend this period?