I don't think there is a longest time between coats. Years maybe?

Yeah unless somebody chimes in otherwise, I'm gonna assume you can't wait even overnight to re-coat it without buffing/roughing up, based on the 4hr re-coat and 24hr ready-to-use instructions. Once a day at night at the same time is probably a better schedule anyway.

As far as Helmsman Spar Urethane goes, Minwax specifies a minimum drying time of only 4 hours before re-coating. I've done this on wraps with no issue (with and without sanding), but I dilute their urethane spar with 30% (volume for volume) with mineral spirits. I should say I have even used 50/50 dilution of Helmsman and it dries fast and hard, but at that dilution it is very thin and runs if the rod is not perfectly level. I'm new to using Helmsman spar urethane as I have traditionally used phenolic or phenolic-modified alkyd resin spars for all of my previous work (e.g. Interlux Schooner and more recently Epifanes Clear). I do find Helmsman, which I understand to contain polyurethane-modified alkyd resin, does dry much faster and harder than traditional spars mentioned above.

I can confirm here that Epifanes does recommend diluting their Clear Vernice varnish up to 50% with naphtha (or their own Brush thinner) and that's what I have used in the recent past for wraps and for dipping cane rods. However even with significant dilution, these traditional spars still seem thick, take at least 24 hrs drying time between coats, and yield softer overall finish than Helmsman urethane spar. The issue is that traditional linseed or (more expensive) tung oil-based spars - regardless of whether they contain phenolic or alkyd (or usually both) resins - also contain high oil:resin ratios making them long oil spars. Long-oil spars are meant to be somewhat soft to allow the finish to remain more flexible with changing environmental conditions. If you want a harder (but inherently less flexible) finish, you need to increase the resin content of the varnish to allow for greater cross-linking of the resins.
FYI I've seen varnishes classified as: short-oil (0.5-1.5 parts oil to 1 part resin), medium-oil (1.5-2.5 parts oil), and long-oil (2.5-5.0 parts oil) or similarly: short-oil containing 5 to 12 gal of oil per 100 lbs of resin, medium-oil containing 12 to 40 gal of oil per 100 lbs of resin, and long oil containing 40 to 100 gal of oil per 100 lbs of resin.

FWIW I think ideally, a medium oil varnish would be suitable for wraps and rod finishing but I am not aware of any commercially available products labelled as such. I can't confirm this but given the fact McCloskey's MOW seems to yield a harder finish than other long-oil varnishes (like Epifanes and the like) it might just be a medium-oil varnish. Too bad I can't get MOW in Canada or else I would experiment with it.
Having said that, there is great advice in this thread (above) and it's worth experimenting with Helmsman or other products if they are available in your area.

I’m a big fan of Helmsman Spar Urethane. I’ve been using the same can for about 2 years and it’s thickened up slightly. I get great coverage with 4 coats and usually let it dry overnight between coats. No prep whatsoever and no issues on a variety of rods including saltwater spinning.

started using Epifanes marine varnish after considerable research..
first impressions are good, like it a lot, but it does need to be thinned more than other varnishes I've used. I warm it and thin 50% with naptha for first coats.

from a site called woodwhisperer.com,
All varnishes are some combination of oils, resins, solvents.
Oils – Linseed Oil or Tung Oil
Resins – Alkyd, Phenolic, or Polyurethane
Solvents – Mineral Spirits, Naptha, or Paint Thinner

Epifanes is tung oil, phenolic resin, naptha and a bit of xylene.
Linseed is cheaper but most of the better marine varnishes use tung.

All that said, I have several rods finished with Ace Spar varnish. Suspect it is linseed and polyurethane, the cheaper ingredients - but the finish looks fine and so far has lasted several years with no problems.
I used this same varnish on some wooden garden furniture, typically would last 3-5 years exposed to all-day sun, wind, rain and whatever else Colorado could serve up. So I fully expect it to last long past my lifetime on a fishing rod I don't get to fish nearly often enough ;-)