So with modern name brand multi stage onboard chargers how often do you check electrolyte level,, weeks/months? Also considering they are new flooded lead acid batteries plugged in full time. Do some folks just plug in for a few days every few weeks? My boat may go months with no use.

I check my traditional lead acid flooded cell batteries levels about every three months and seldom do they need water. I charge mine up when I get home and then after a day or two I disconnect the onboard charger. The night prior to using the boat, I will plug it back in just to make sure the batteries are topped off. If not using the boat for an extended period of time, I plug in the charger about once a month for an overnight charge.

I check mine once a month, I leave mine plugged in. Never had an issue with a battery.

What I have been told and what I have always done.

Check monthly. If getting low, charge, top off, recharge.

Check with charger mfg. on leaving on for long periods of time.

Lead acid flooded cell batteries should be recharged at least monthly.

My crappie Interstates need about a quart a month. Mounted inside a center console bay boat makes topping them off a pia. Definitely have some AGM's on my Christmas list.

Love my aGM and never worry about them and they last 5 to 7 years guiding.

Leave mine plugged in always. Have to add water about every 2 months.

Switching to lithium right now, no more water level checks...

Check the batteries when you go around and check everything else, such as bolts holding trolling motor to boat, bolts holding motor to jackplate, bolts holding jackplate to boat, bolts holding shallow water anchors to mounts and mounts to jackplate. Check and clean all battery connections and at that time check the water. I would do this apx every 6 weeks, more if you're on the lake several times a month.
On the charger, that is more a personal choice. Myself, I like to charge when I come off the lake, then disconnect after 24 to 48hrs. Recharge/top off the nite before going fishing the next day. Chargers monitor the batteries and kick on when the battery is at a certain percent discharge. This means that if the discharged amount is just before time for the charger to kick on, and you take it off charge, then you're not going out with 100% battery.
John