I don't know any plants that grow continuously, forever. Most grow in spurts followed by rest periods. The leaves of figs last longer than a year, but not forever. I don't know how long elastica leaves last, maybe 2 to 3 or more years. It probably varies by situation, but the basic idea is that as new leaves emerge, the eldest and/or least productive are cut off from the cycle of fluids from the roots flowing to the leaves being converted in the presence of sunlight into sugars and returned to the roots. In ideal conditions, like outdoors in Indonesia, more new leaves are growing than are used-up so the tree gets bigger, faster, and the growth cycles are longer and rest cycles are shorter. Indoors in the UK are whatever they are, but you will see them grow, rest, grow, rest, cycles not related to what we would consider normal growth cycles of grow in spring when sunlight is increasing, rest in autumn and winter.
Equatorial plants have increasing sunlight levels two times a year, and two summers, and two winters such as they are! At the beginning of the year, December 21st the sun is over the Tropic of Capricorn the length of the day begins to increase. On June 21st, the other winter of the Equator, the sun is over the Tropic of Cancer and the length of day begins to increase, again. Equatorial plants never have less sunlight than we do the 1st day of spring. Most plants respond to increasing sunlight by growing, and your rubber tree is programmed to have two strong growing periods per year, with corresponding short quiet periods. You should expect your tree to have two growth spurts per year. I should point out that while they don't have "winter" at the equator, they do commonly have two seasons: wet and dry based upon the direction of the wind which blows east part of the year and west the other part. In that situation rubber trees grow in wet and rest in dry. Exactly how dry, and how long dry depends upon geography.
You can expect that the interior leaves and the lower leaves are the eldest, and that leaves on the shady side are less productive than the sunny side. Those are the leaves that will turn ugly and fall off, one or two at a time, usually in conjunction with new leaves expanding at the tips of branches. Only rotate the tree two times a year, after each growth spurt. Make it grow more compact by cutting off the bud at branch tips plus the 1st leaf, just above the axil of the 2nd leaf. Only feed during the growth period, not otherwise. Continue your watering schedule, it looks just fine.
soon. Add some good compost and water it well.
