Courageous' numbers are a bit optimistic! If any rubbish gets in the pump strainer, or the output hose is not perfect size/kinked etc. In those cases, the pump continues to work at the same power but moves less liquid. Lets say it only actually moves 1/2 what you expect (I don't know how Rule pumps specs compare to reality), but the pumps I buy in my dayjob usually are optimistically named.
If you regularly let your battery discharge by as much as 30 hours at 2 A (ie. 60 Ah) on an 85 Ah battery it won't last long (especially if it is a normal battery rather than a deep cycle). Normal advice is not to discharge by more that 50%. Lets assume that your battery is 85% charged when it starts (after a long trip it might be more than this, but lets play safe). That means you have 35% of its capacity before you reach its limits. That is 30 Ah which at 2A for the pump is 15 hours of pumping.
So that is 3750 gallons. Assuming your boat is kept in Cornwall as per your profile, then the annual average rainfall
is about 1m (bear in mind this is average so some years will be worse than others) and the worst months are about 0.12m. For simplicity lets assume your boat is rectangular 5m x 2.3m and so in a year will gather 11.5 m3 of rain. If you add in lets say 1 gal per day of spray or slight leaks coming aboard then you will have an annual total of about 3500 US Gallons of water coming on board!
So that says your battery, should manage
about a whole year pumping out your boat. But there are a few other things to think about that will zap the battery:
(1) temperature. the battery works less efficiently at cold temps, but self discharges faster at high temp!
(2) the natural self discharge of the battery which will be about 40+% per annum at UK temps - so about 3-4% per month.
(3) the automatic sensor in your pump draws power even when off (compared to a float switch system which does not). It uses something like 1% of the rated current to check if there is any water to pump. So that is 0.02A which sounds tiny, but that is 0.48 Ah per day.
If you are lucky your solar panel will top up the losses. But you don't say which panel you are using and I expect you will be lucky to get an average of 0.1 Ah from small panels in typical UK sun unless you have an expensive panel and mount it at just the right angle etc. If your boat is only afloat in summer then you are getting over 150 hrs of sunshime a month, so probably about enough to make up for the losses above. But in the depths of winter you are getting 50 hrs a month of sunshine at best - and of course that corresponds to maximum rainfall too.
All together if it was me I wouldn't be leaving it without at least a quick run and checking the battery voltage for more than 4-6 wks during the summer; and probably wouldn't leave it for more than a couple of weeks in winter.