Can manic episodes in bipolar disorder become more frequent and intense and severe as the disease progresses?

Not progressive: Bipolar disorder is not usually thought of as a progressive disease. Manic episodes and mood cycling are more problematic when not treated effectively. Common treatments usually include a mood stabilizer, such as lithium carbonate, and psychotherapy. The combination of the two is usually most effective.

Probably so w/o Rx: Bipolar disorder is believed to exhibit a 'kindling' kind of phenomenon, where each untreated mood episode (it doesn't matter if it is manic or depressive) is likely to increase the probability that the next mood episode will occur with a shorter interval than the last one and will last longer. Again, this presumes no treatment.