How often does a pen actually need flushing, and what does neglect look like before it becomes a problem?

What accumulates is mostly dried ink in the feed's channels, plus whatever the ink left behind as water evaporated. That is why the answer depends on the ink rather than on the calendar, and why every schedule you have read is right for somebody.

The early signs, in the order they usually appear:

1. Slower start after a rest. Yours is doing this. A pen that used to write instantly after two days and now needs a moment is telling you the feed is not holding as much as it did.
2. Reduced flow. The line gets drier and thinner at the same nib and paper. Your other pen.
3. Colour contamination. New ink comes out slightly wrong for the first page.
4. Hard starts every time, then skipping mid-word. By here it is a real problem rather than a signal.

You are at stage one and two, which is exactly the right moment.

A workable rule rather than a schedule you will not keep: flush whenever you change ink, and additionally if a pen has sat unused for a few weeks with ink in it. For a pen used constantly with one well-behaved ink, that can genuinely be a few times a year.

The method is dull: flush with cool water until it runs clear, then keep going a bit longer than you think, then let it dry properly nib-down on a cloth. Cool, not hot — heat is what damages the plastic parts and any adhesive holding the nib.

On escalation, since you will eventually meet a pen that a plain flush does not fix.

A drop of dish soap in the water is the next step and it handles most stubborn cases. Rinse thoroughly afterwards — soap residue causes its own flow oddities and people then conclude the cleaning made things worse.

A pen flush solution is a step beyond that and is worth having if you own several pens.

Soaking overnight in cool water solves a surprising amount and requires no skill.

An ultrasonic cleaner — I own one and I would not recommend one to you. It is genuinely effective and it is also the fastest way to loosen a nib that was set with adhesive, damage a decorated cap, or wreck a vintage pen with a fragile sac. If you use one, it is for the nib and feed only, never the whole pen, and never with heat.

What I would avoid entirely: hot water, alcohol, and anything ammonia-based on a pen you care about. All three are recommended somewhere, all three will be fine on some pens, and all three are how people damage plastics and finishes that were fine before they started.

Since nothing is obviously wrong with yours, plain water is very likely all you need. Save the escalation for a pen that has actually stopped.

The variable nobody mentions in the frequency arguments is which ink is in the pen, and it changes the answer by an order of magnitude.

Roughly, from least to most demanding:

Ordinary dye-based inks from the major makers. Well behaved, designed to be forgiving. A pen with one of these in constant use can go a long time between cleanings.

Heavily saturated and shimmering inks. Shimmer is suspended particles, and particles settle in narrow channels. These want cleaning more often and want the pen used regularly, or the particles have time to settle where you cannot reach.

Iron gall and pigmented inks. These are the ones that punish neglect. Flush promptly, do not leave them sitting unused for long, and follow the maker's guidance rather than general advice.

Anything unusual — home-mixed, very old, or a brand nobody discusses — treat as demanding until proven otherwise.

Which also answers your six-pen situation: you probably do not need to treat them all the same. The one with the plain dye ink you use daily can wait. If any of them has something shimmering or pigmented in it, that is the one to do this week.

And the single habit that prevents most of this: do not leave a pen inked and unused for months. Ink left to dry slowly is what causes the problems people then need aggressive cleaning to fix. If a pen is going into a drawer, empty it first.