Depending on the species of cactus and your climate, how long a cactus should spend indoors varies. Although they're famous for being tough when older, cactuses are often sensitive as seedlings and need protection from strong sun and other elements, so acclimate them to the outdoors slowly and only when they are ready.
Seed Germination Times
The time it takes for cactus seed to germinate depends on the species and where you are growing it. When you start cactuses indoors, the conditions are usually more controlled than unpredictable outdoor weather, so cactus seeds germinate more readily, any time between three weeks and several months. Outdoors, on the other hand, cactuses will wait for ideal temperature and moisture conditions to germinate, which may take several years. Indoors, they usually germinate best sprinkled in the topmost layer of a tray of moistened cactus soil, placed in a lightly shaded windowsill with a transparent lid over the top.
Once cactus seedlings begin to develop, you can gradually begin to remove the moisture-conserving transparent cover. This time varies widely depending on species; the best indicator that you may safely remove it is the development of spines on the seedlings. Take several months to remove the cover all the way, leaving it off for longer periods or cutting pieces out of the center to avoid changing the environment too abruptly. Wait until the seedlings are about the size of a marble, roughly six months or so -- though again, size is a better indicator -- to plant them into their own pots.
Cactus seedlings usually need some sun protection for the first few years or until the seedlings are about 3 inches across. Generally a lightly shaded southern windowsill is the best starting point for cactus seedlings, and you can begin to introduce them to sun gradually when they are about 1 inch across. Increase light levels slowly, stopping if the seedlings show any signs of turning red or scorching. Once you can consistently leave cactuses in full sun on a windowsill, you can begin to move them outside.
How cactuses fare outdoors depends on their hardiness. Prickly pear cactus (Opuntia compressa), for example, is a relatively cold-tolerant species that can grow in U.S. Department of Agriculture plant hardiness zones 4 through 9 and will adjust to cool weather faster than, say, Christmas cactus (Schlumbergera truncata), which can grow outdoors only in the frost-free climates of USDA zones 10 through 12. Before putting a cactus outdoors permanently or even in a pot on the patio, check its hardiness range and wait for warm enough weather.
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