Bevacizumab is discontinued as a result of toxicity in only a small proportion of patients and most patients who receive bevacizumab can be treated until progression. It is far more likely that chemotherapy will be interrupted due to toxicity, for example, oxaliplatin-induced neuropathy, or irinotecan-induced neutropenia. The AVF2107g study of irinotecan-based chemotherapy with or without bevacizumab reported 7.1% of patients discontinuing study treatment due to adverse events in the chemotherapy arm and 8.4% in the chemotherapy plus bevacizumab arm [Hurwitz, et al. NEJM 2004].
In the NO16966 study 15% and 21% of patients discontinued treatment due to grade 3/4 adverse events in the XELOX/FOLFOX4 and XELOX/FOLFOX4 plus bevacizumab arms, respectively. Importantly, the proportion of patients discontinuing treatment due to adverse events of special interest to bevacizumab, e.g. hypertension, was only 2% in the chemotherapy alone arm and 5% in the bevacizumab containing arm [Saltz, et al. JCO 2008].

Several trials are currently investigating this issue. The TML trial (NCT00700102) is designed to assess bevacizumab-based therapy as a second-line treatment in mCRC patients after progression following a first-line bevacizumab-containing regimen. Recruitment for this trial is now complete, and the results are expected to be presented at ASCO 2012.
A trial investigating the use of bevacizumab beyond progression in patients with mBC (TANIA, NCT01250379) began recruiting in February 2011 and is expected to complete recruitment around August 2013. This treatment approach is also being investigated in advanced NSCLC in the AvaALL trial (NCT01351415) which is expected to be completed in 2014.

Yes, results from studies of patients with both lung and ovarian cancer support the use of bevacizumab until progression. The ECOG 4599 study examined bevacizumab in combination with carboplatin plus paclitaxel until disease progression in NSCLC patients. A significant survival benefit was observed in patients receiving bevacizumab compared to those treated with chemotherapy alone. This survival benefit was also apparent in patients who continued to receive bevacizumab plus chemotherapy compared to chemotherapy alone in the absence of disease progression [Sandler, et al. WCLC 2011]. Also, the SAiL study showed a median overall survival of 14.6 months in a broad population of patients with advanced, metastatic or recurrent non-squamous NSCLC who received bevacizumab until disease progression [Crino, et al. Lancet Oncology 2010] and a retrospective analysis of the US Oncology network's electronic medical records demonstrated treatment with bevacizumab until progression led to more favourable clinical outcomes [Nadler, et al. Oncologist  2011].
The GOG-0218 trial (NCT00262847) investigated the effect of treating patients with advanced ovarian cancer with chemotherapy alone or in combination with bevacizumab for 4 or 15 months. There was a significant progression-free survival benefit for patients receiving bevacizumab for 15 months compared to chemotherapy alone (14.1 versus 10.3 months, respectively) [Burger, et al. ASCO 2010].