I'm 55 yrs old and just bought a rowing machine. I row for 2 1/2 minutes very hard (very winded!) 3 to 5 times a day. Is this enough or should I be doing more?
First, congrats on your purchase of a Concept2 Indoor Rower. I, too, own that same rower. In my opinion, you've purchased the gold standard for rowers. With any training program, it's important to use the best equipment if you're serious about achieving your goals.
Now, as for your goals. Your current approach is not optimal for your stated goals. Rather than performing a series of brief, intense rows, you should consider switching to Steady State rows. “Steady State” can mean different things to different people, but, in this context, it implies longer rows at a consistent stroke rating (eg. 18-20 spm). You won't need to totally abandon interval training, but, the majority of your rows should be steady state.
“From my experience, the biggest mistake people make with the erg is that they only, or mostly, use it for testing and hard interval training. The erg becomes a painful experience and we learn to dislike it, often to the point of being afraid of it.
Instead, try to spend time on the erg doing steady state. I find the more I erg, the more I get into a rhythm and the easier, both mentally and physically, it becomes. Listen to music, books on tape, watch TV, but simply, spend time on the erg.”
While your goal to row 2k under 8 minutes is somewhat ambitious for a rowing “newbie”, using steady state training will assist you in preparing for that goal. Steady state training will help build a base of fitness by training your heart to be more efficient over longer distances. In addition, there's the added benefit of weight control since you should be training within your aerobic zone.
Rowing a measured 'piece' (eg. 2k) requires correct form. Contrary to popular opinion, rowing is not a pulling sport. Rather, it's a pushing sport since the first movement of the rowing stroke is to engage the legs to push against the foot plates. If you haven't already done so, I would strongly recommend you take a look at the C2 technique videos to make sure you start with correct form. Using correct form will assist you in achieving any rowing goal. In addition, take a moment to read up on Damper Setting 101 and Debunking the Myths: Damper Setting, Stroke Rate, and Intensity. Those articles will help you avoid many common mistakes made by first time users of the C2 rower.
Lastly, as you progress towards your goal(s), there are lots of resources available to you. Depending on the C2 model you own, you can track your progress using the on-board performance monitor and log the results in the C2 Logbook. The logbook provides a history of your training sessions. Also, there's daily workouts available so you can maximize your time on the rower.
Good luck and have fun!
Just based on age, exercise response and the fact this piece of equipment is new to you - just keep working up your intensity. There isn't a "set time" per se. For now just stick to what you can do.
As @rriower said as you progress you should look to start a regimented plan (based on your goals etc).